I'm having similar issues (pro version) with the UICamera and the MainCamera playing nicely. World is rendering correctly but I'm not getting my character health bars displaying over their heads (like the Workshop demo). When I deactivate the MainCamera, the health bars show up and I can still see last frame rendered by the MainCamera (only clears depth so this is expected).
MainCamera is dynamically added to scene. Clear flags: Skybox. Depth 0. Perspective. Far clip: 2000.
UICamera is statically added to scene. Clear flags: Depth Only. Depth 0. Orthographic. Far clip: 2.
Edit: OK, this is odd. While I'm playing, if I change the UICamera depth to a non-zero number, THEN change it back to 0, everything renders fine.