Before I went to using the Oculus Rift I used Windowed mode exclusively. It allowed me to Alt+Tab with far less crashed to Desktop. In order to activate it you need to go into the graphical setting in the launcher. Click on the gear icon in the left column. Then you click on the graphical option. That should pop up a window with the basic options like resolution, AA, PP, Etc. In that menu there is an option for Full Screen or windowed mode. Click to Windowed mode and click apply. Then load up a multiplayer or singleplayer game as usual.
My instructions might not have the option names quite correct as I dont have rF2 in front of me right now.