Are the two cameras you're using being used only for your UI, or do they display something else (like parts of a scene)? Unless you want to finagle your z-depths around, I'd strongly suggest creating a dedicated camera (or cameras) for the UI elements so they can always be drawn on top of everything else.
Personally, I'd use a single camera for the UI and just place buttons on both the left and right side. Then those buttons would be batched to a single draw call (two draw calls if you use labels with fonts).