products with built in COLLATORS
This is a professional item and cost lots and take lots of room.
With regard normal printing you can print progressively 1 complete document at a time then repeat .[print evens then turn and print odds]
You can also with some printer reverse the print order last page first so that it is delivered ready for the next stage.
Within the Laser printer range there is the opportunity to multiplex[print both side simultaneously]- but this does not happen with ink jet machines I think.