I have never tried what your are describing. So I just tested it.
Is this what you are seeing;
1) Pi boots up, X Window loads on HDMI. Keyboard and mouse work within this X Windows Session.
2) From the terminal within the above X Windows session, you enter FRAMEBUFFER=/dev/fb1 startx
3) X Windows loads into PiScreen and now mouse and keyboard only work on X windows session in PiScreen
And then you want to close down the X Windows session on PiScreen to return to X Windows on the HDMI?
If so, have you tried logging out of the PiScreen X Windows session?
Mark --OzzMaker.com --