Flicker between one roScreen and another roScreen

I don't think you can. Creating an roScreen causes the screen to go black until you call SwapBuffers(). What I usually do in a situation where I need to use two different roScreens is I'll do a quick fade to black on the old screen and a fade in on the new screen, so it's not quite so jarring. The other option would be to use the same roScreen and either store it globally or pass it back and forth to the functions that need it.