Hi Karen, I’m not sure what the problem is, when you set your small 3D viewport window to rendered view, you can go into Edit Mode, Sculpt; Texture Paint in the main 3D Viewport, or do whatever you like and the small window will stay (‘pinned’) in rendered view. You don’t actually need to ‘pin’ that window (and even can’t, like you already observed):


With Texture Paint, I would use Preview Mode (formerly known as LookDev), or Eevee, because Cycles doesn’t update the texture.
Is this what you meant?