An implementation to use DuoTone Icons in Xamarin.Forms

DuoToneIcon.cs

using System;
using System.Collections.Generic;
using System.Globalization;
using System.Text;
using System.Text.RegularExpressions;
using Xamarin.Forms;

namespace Fontawesome
{
    public class DuoToneIcon : AbsoluteLayout
    {
        public EncodingStyle Encoding { get; set; } = EncodingStyle.Unicode;
        public enum EncodingStyle
        {
            Unicode,
            IconName
        }
        /// <summary>
        /// MANDATORY ON ANDROID AND UWP
        /// The font file name in your project including the file ending
        /// e.g. FontAwesome5Duotone.otf
        /// </summary>
        public string FontFileName { get; set; }
        private string FontFamily { get; set; }
        private Label primaryLabel = new Label();
        private Label secondaryLabel = new Label();
        #region LabelProperties
        public TextAlignment HorizontalIconAlignment { get { return (TextAlignment)GetValue(HorizontalIconAlignmentProperty); } set { SetValue(HorizontalIconAlignmentProperty, value); } }
        public static readonly BindableProperty HorizontalIconAlignmentProperty = BindableProperty.Create("HorizontalTextAlignment", typeof(TextAlignment), typeof(DuoToneIcon), TextAlignment.Start, propertyChanged: OnHorizontalIconAlignmentPropertyChanged);
        private static void OnHorizontalIconAlignmentPropertyChanged(BindableObject bindable, object oldValue, object newValue)
        {
            var control = (DuoToneIcon)bindable;
            control.primaryLabel.HorizontalTextAlignment = (TextAlignment)newValue;
            control.secondaryLabel.HorizontalTextAlignment = (TextAlignment)newValue;
        }
        public TextAlignment VerticalIconAlignment { get { return (TextAlignment)GetValue(VerticalIconAlignmentProperty); } set { SetValue(VerticalIconAlignmentProperty, value); } }
        public static readonly BindableProperty VerticalIconAlignmentProperty = BindableProperty.Create("VerticalTextAlignment", typeof(TextAlignment), typeof(DuoToneIcon), TextAlignment.Start, propertyChanged: OnVerticalIconAlignmentPropertyChanged);
        private static void OnVerticalIconAlignmentPropertyChanged(BindableObject bindable, object oldValue, object newValue)
        {
            var control = (DuoToneIcon)bindable;
            control.primaryLabel.VerticalTextAlignment = (TextAlignment)newValue;
            control.secondaryLabel.VerticalTextAlignment = (TextAlignment)newValue;
        }
        public double IconSize { get { return (double)GetValue(IconSizeProperty); } set { SetValue(IconSizeProperty, value); } }
        public static readonly BindableProperty IconSizeProperty = BindableProperty.Create("IconSize", typeof(double), typeof(DuoToneIcon), 12d, propertyChanged: OnIconSizePropertyChanged);
        private static void OnIconSizePropertyChanged(BindableObject bindable, object oldValue, object newValue)
        {
            var control = (DuoToneIcon)bindable;
            control.primaryLabel.FontSize = (double)newValue;
            control.secondaryLabel.FontSize = (double)newValue;
        }
        public Color PrimaryColor { get { return (Color)GetValue(PrimaryColorProperty); } set { SetValue(PrimaryColorProperty, value); } }
        public static readonly BindableProperty PrimaryColorProperty = BindableProperty.Create("PrimaryColor", typeof(Color), typeof(DuoToneIcon), Color.White, propertyChanged: OnPrimaryColorPropertyChanged);
        private static void OnPrimaryColorPropertyChanged(BindableObject bindable, object oldValue, object newValue)
        {
            var control = (DuoToneIcon)bindable;
            control.primaryLabel.TextColor = (Color)newValue;
        }
        public Color SecondaryColor { get { return (Color)GetValue(SecondaryColorProperty); } set { SetValue(SecondaryColorProperty, value); } }
        public static readonly BindableProperty SecondaryColorProperty = BindableProperty.Create("SecondaryColor", typeof(Color), typeof(DuoToneIcon), Color.White, propertyChanged: OnSecondaryColorPropertyChanged);
        private static void OnSecondaryColorPropertyChanged(BindableObject bindable, object oldValue, object newValue)
        {
            var control = (DuoToneIcon)bindable;
            control.secondaryLabel.TextColor = (Color)newValue;
        }
        public double PrimaryOpacity { get { return (double)GetValue(PrimaryOpacityProperty); } set { SetValue(PrimaryOpacityProperty, value); } }
        public static readonly BindableProperty PrimaryOpacityProperty = BindableProperty.Create("PrimaryOpacity", typeof(double), typeof(DuoToneIcon), 1d, propertyChanged: OnPrimaryOpacityPropertyChanged);
        private static void OnPrimaryOpacityPropertyChanged(BindableObject bindable, object oldValue, object newValue)
        {
            var control = (DuoToneIcon)bindable;
            control.primaryLabel.Opacity = (double)newValue;
        }
        public double SecondaryOpacity { get { return (double)GetValue(SecondaryOpacityProperty); } set { SetValue(SecondaryOpacityProperty, value); } }
        public static readonly BindableProperty SecondaryOpacityProperty = BindableProperty.Create("SecondaryOpacity", typeof(double), typeof(DuoToneIcon), 0.5d, propertyChanged: OnSecondaryOpacityPropertyChanged);
        private static void OnSecondaryOpacityPropertyChanged(BindableObject bindable, object oldValue, object newValue)
        {
            var control = (DuoToneIcon)bindable;
            control.secondaryLabel.Opacity = (double)newValue;
        }
        #endregion
        public string Icon { get { return (string)GetValue(IconProperty); } set { SetValue(IconProperty, value); } }
        public static readonly BindableProperty IconProperty = BindableProperty.Create("Icon", typeof(string), typeof(DuoToneIcon), "", propertyChanged: OnIconPropertyChanged);
        private static void OnIconPropertyChanged(BindableObject bindable, object oldValue, object newValue)
        {
            var control = (DuoToneIcon)bindable;
            control.primaryLabel.Text = control.Encoding == EncodingStyle.Unicode ? UnicodeToCharacter(newValue.ToString()) : newValue.ToString() + "-primary";
            control.secondaryLabel.Text = control.Encoding == EncodingStyle.Unicode ? UnicodeToCharacter("10" + newValue.ToString()) : newValue.ToString() + "-secondary";
        }
        private static string UnicodeToCharacter(string inStr)
        {
            int code = 0;
            bool success = int.TryParse(inStr, System.Globalization.NumberStyles.HexNumber, CultureInfo.InvariantCulture, out code);
            if (success)
            {
                return char.ConvertFromUtf32(code);
            }
            return inStr;
        }
        public DuoToneIcon()
        {
            switch (Device.RuntimePlatform)
            {
                case Device.iOS:
                    FontFamily = "FontAwesome5Duotone-Solid";
                    break;
                case Device.UWP:
                    FontFamily = "/Assets/" + FontFileName + "#Font Awesome 5 Duotone";
                    break;
                case Device.Tizen:
                    FontFamily = "FontAwesome5Duotone";
                    break;
                case Device.Android:
                    FontFamily = FontFileName + "#Regular";
                    break;
                default:
                    FontFamily = "FontAwesome5Duotone";
                    break;
            }
            primaryLabel.FontFamily = FontFamily;
            primaryLabel.Text = Encoding == EncodingStyle.Unicode ? Icon : Icon + "-primary";
            primaryLabel.TextColor = PrimaryColor;
            primaryLabel.Opacity = PrimaryOpacity;
            primaryLabel.FontSize = IconSize;
            primaryLabel.HorizontalTextAlignment = HorizontalIconAlignment;
            primaryLabel.VerticalTextAlignment = VerticalIconAlignment;
            secondaryLabel.FontFamily = FontFamily;
            secondaryLabel.Text = Encoding == EncodingStyle.Unicode ? "10" + Icon : Icon + "-secondary";
            secondaryLabel.TextColor = SecondaryColor;
            secondaryLabel.Opacity = SecondaryOpacity;
            secondaryLabel.FontSize = IconSize;
            secondaryLabel.HorizontalTextAlignment = HorizontalIconAlignment;
            secondaryLabel.VerticalTextAlignment = VerticalIconAlignment;
            AbsoluteLayout.SetLayoutBounds(primaryLabel, new Rectangle(0, 0, 1, 1));
            AbsoluteLayout.SetLayoutFlags(primaryLabel, AbsoluteLayoutFlags.All);
            AbsoluteLayout.SetLayoutBounds(secondaryLabel, new Rectangle(0, 0, 1, 1));
            AbsoluteLayout.SetLayoutFlags(secondaryLabel, AbsoluteLayoutFlags.All);
            this.Children.Add(secondaryLabel);
            this.Children.Add(primaryLabel);
        }
    }
}

Usage:
<fontawesome:DuoToneIcon Icon="f6be" Encoding="Unicode"/>
OR
<fontawesome:DuoToneIcon Icon="cat" Encoding="IconName"/>

Cheatsheet : https://fontawesome.com/cheatsheet/pro/duotone