Shift gear from 2 sliders?

Here's a hack to achieve what you want:

1. Create a new Slider

2. Drop the script below on the GameObject named Handle (not Handle Slide Area)

3. Duplicate the GameObject named Background and rotate it 90° just like you probably did in the picture (just for the looks).

4. You may want to disable the Fill Area (also for the looks)

5. ???

6. Profit

ShiftGearHack.cs :

 using UnityEngine;
 using UnityEngine.EventSystems;
 using UnityEngine.UI;
 
 [RequireComponent(typeof(Image))]
 public class ShiftGearHack : MonoBehaviour, IBeginDragHandler, IDragHandler, IEndDragHandler
 {    
     Slider slider;
     
     public void OnBeginDrag(PointerEventData eventData)
     {
         var canvas = FindInParents<Canvas>(gameObject);
         if (canvas == null)
             return;
         
         SetDraggedPosition(eventData);
         slider = FindInParents<Slider>(gameObject);
     }
     
     public void OnDrag(PointerEventData data)
     {
         slider.OnDrag (data);
         SetDraggedPosition(data);
     }
     
     private void SetDraggedPosition(PointerEventData data)
     {
         Vector2 globalMousePos;
 
         if (RectTransformUtility.ScreenPointToLocalPointInRectangle(transform.parent as RectTransform, data.position, data.pressEventCamera, out globalMousePos))
         {
             if (transform.localPosition.sqrMagnitude < 5f*5f) {
                 slider.value = (slider.minValue + slider.maxValue) * 0.5f;
 
                 float horizDelta = Mathf.Abs (globalMousePos.x - transform.localPosition.x);
                 float vertDelta = Mathf.Abs (globalMousePos.y - transform.localPosition.y);
                 if (horizDelta > 5f && horizDelta > vertDelta) {
                     slider.SetDirection (Slider.Direction.LeftToRight, true);
                 }
                 else if (vertDelta > 5f && vertDelta > horizDelta) {
                     slider.SetDirection (Slider.Direction.TopToBottom, true);
                 }
             }
         }
     }
     
     public void OnEndDrag(PointerEventData eventData)
     {
     }
     
     static public T FindInParents<T>(GameObject go) where T : Component
     {
         if (go == null) return null;
         var comp = go.GetComponent<T>();
         
         if (comp != null)
             return comp;
         
         Transform t = go.transform.parent;
         while (t != null && comp == null)
         {
             comp = t.gameObject.GetComponent<T>();
             t = t.parent;
         }
         return comp;
     }
 }