An AttachedProperty that lets you display a different windows in the same process in different taskbar groups in windows.

TaskbarHelper.cs

using System;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;
using System.Windows;
using System.Windows.Interop;

namespace TasbarSample
{
    public static class TaskbarHelper
    {
        public static string GetApplicationUserModelId(DependencyObject obj) => (string)obj.GetValue(ApplicationUserModelIdProperty);
        public static void SetApplicationUserModelId(DependencyObject obj, string value) => obj.SetValue(ApplicationUserModelIdProperty, value);

        public static readonly DependencyProperty ApplicationUserModelIdProperty =
            DependencyProperty.RegisterAttached("ApplicationUserModelId", typeof(string), typeof(TaskbarHelper), new PropertyMetadata(null, ApplicationUserModelIdCahnged));

        private static void ApplicationUserModelIdCahnged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            if (!(d is Window window))
                return;
            if (!(e.NewValue is string applicationUserModelId))
                return;

            window.Loaded += (s, eargs) =>
            {
                var handle = new WindowInteropHelper(window).Handle;
                IPropertyStore propertyStore = null;
                SHGetPropertyStoreForWindow(handle, ref IID_PropertyStore, ref propertyStore);

                var key = new PROPERTYKEY() { pid = 5, fmtid = new Guid("9F4C2855-9F79-4B39-A8D0-E1D42DE1D5F3") };

                object id = applicationUserModelId;
                propertyStore.SetValue(ref key, ref id);
            };
        }

        public static Guid IID_PropertyStore = new Guid("886d8eeb-8cf2-4446-8d02-cdba1dbdcf99");

        [StructLayout(LayoutKind.Sequential, Pack = 4)]
        public struct PROPERTYKEY
        {
            public Guid fmtid;
            public uint pid;
        }

        [ComImport, Guid("886D8EEB-8CF2-4446-8D02-CDBA1DBDCF99"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
        public interface IPropertyStore
        {
            [MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime)]
            void GetCount([Out] out uint cProps);

            [MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime)]
            void GetAt([In] uint iProp, out PROPERTYKEY pkey);

            [MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime)]
            void GetValue([In] ref PROPERTYKEY key, out object pv);

            [MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime)]
            void SetValue([In] ref PROPERTYKEY key, [In] ref object pv);

            [MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime)]
            void Commit();
        }

        [DllImport("Shell32.dll")]
        private static extern uint SHGetPropertyStoreForWindow(IntPtr hwnd, ref Guid riid, ref IPropertyStore ppv);

        [DllImport("Shell32.dll")]
        private static extern uint GetCurrentProcessExplicitAppUserModelID(out string appID);
    }
}