You mean, scrolling continuously, a la Super Mario Brothers or Gradius? =)

As stuff scrolls off the screen to the left, replace it. That way the third screen will appear as the second one leaves, and you don't have to put the world on hold once the first screen is gone to replace the entire screen at once (which would be likely to spill over into the non-VBLANK period, causing much wailing and gnashing of teeth).