99cbe960_ackBerry_PlayBook_Tablet___2_1__Guideline
[Characteristic] One screen appears at a time [Guideline] 
Use a single screen if possible. If your application requires multiple screens to be open at the same time, use a split screen or rethink the flow of your application.
Constraints of designing for mobile devices - Link - Find the latest UI Guidelines for the BlackBerry PlayBook Tablet - 2.1