Unity Target Group weight change

TargetGroupBlender.cs

/* Blend to camera focus from the character to the firefly.
 * No effect on actual target group weighting.
 */
using System.Collections;
using Cinemachine;
using UnityEngine;

namespace Project
{
    [RequireComponent(typeof(CinemachineTargetGroup))]
    public class TargetGroupBlender : MonoBehaviour
    {
        private static readonly YieldInstruction _WAIT_FOR_END_OF_FRAME = new WaitForEndOfFrame();
        private CinemachineTargetGroup.Target _character;
        private CinemachineTargetGroup.Target _firefly;
        private CinemachineTargetGroup _targetGroup;

        private void Start()
        {
            _targetGroup = GetComponent<CinemachineTargetGroup>();
            _character = _targetGroup.m_Targets[0];
            _firefly = _targetGroup.m_Targets[1];
        }

        public void LerpToFireFly(float duration)
        {
            StartCoroutine(LerpToFirefly(duration));
        }

        private IEnumerator LerpToFirefly(float duration)
        {
            var start = Time.time;
            var end = start + duration;
            Debug.Log(new {start, end, now = Time.time});
            for (var characterWeight = 1f; Time.time < end; characterWeight = (end - Time.time) / duration)
            {
                Debug.Log($"{nameof(characterWeight)}:  {characterWeight:F}");
                _character.weight = characterWeight;
                _firefly.weight = 1 - characterWeight;
                yield return _WAIT_FOR_END_OF_FRAME;
            }
        }
    }
}

WorkingTargetBlender

using Cinemachine;
using UnityEngine;

namespace Project.Cinemachine_Target_Groups
{
    [RequireComponent(typeof(CinemachineTargetGroup))]
    public class TargetGroupTransition : MonoBehaviour
    {
        private CinemachineTargetGroup _targetGroup;

        private void Start()
        {
            _targetGroup = GetComponent<CinemachineTargetGroup>();
        }

        private void Update()
        {
            if (Input.GetKeyDown(KeyCode.UpArrow))
            {
                _targetGroup.m_Targets[0].weight = 0;
                _targetGroup.m_Targets[1].weight = 1;
            }


            if (Input.GetKeyDown(KeyCode.DownArrow))
            {
                _targetGroup.m_Targets[1].weight = 0;
                _targetGroup.m_Targets[0].weight = 1;
            }
        }
    }
}