RStudio IDE, though awesome in many ways, does not play nice with MacOS when running multiple instances / windows /sessions. It does not work properly with "App Exposé" (Fn + F3, or swipe down with four fingers) which shows you all of the open windows from the same App. When you use Exposé it merely shows you the most recent window. So, if using mouse gestures, the only way I have found to switch between difference instances of RStudio is to use Mission Control (Fn + F4, or swipe up with four fingers), and then select the appropriate window. This is less convenient because Mission control shows all open windows in the workspace, from all open apps.
You also cannot use CMD+backtick to change the window either. However, I have noticed that you can use CTRL+TAB to access different windows, and instance/session names are shown here. This is pretty useful. Actually this could be more useful than using CMD+backtick.
But it would still be nice to get Exposé to work.