basically, you create a UI canvas specifically for your controls, all elements will be invisible but will react to touch events.
-- make one button that fill the left half of the screen, and another that fill the right half of the screen. RectTransform allows you to create buttons that will automatically adjust layout to fit any screen sizes
-- now, you simply use the UI Proxies to catch UI events like click, drags etc etc.
-- You do know about the Unity UI proxy on the Ecosystem right?