With all due respect, Maximara, Byrne and Levitz co-created the Pocket Universe. While it appeared in the LSH book immediately before Byrne's Superman #8, it was a coordinated effort between the two books that both writers worked together on to establish a patch. They failed. That failure figures largely in how the Pocketverse couldn't handle the likes of Mon-El, Supergirl, or the classic LSH stories involving time travel, or the Computo storyline which linked back to Superman's/Batman's time and that continuity. Both books came out in Aug. 1987. [/i]
I'm not entirely aware of the history of this period, so I really can't comment on if they had the idea at the same time.
But I see much more of Byrne's influence in the Pocket Universe. It involved illogical explanations and shoddy, easily punctured scholarship, and this is Byrne's trademark: remember his ever-so-well-thought-out DOOM PATROL reboot? Pocket Universe also had heroism tarnished by deliberately antisocial actions and minimalization of concepts that better writers made grand (the Superboy world was just a POCKET UNIVERSE all along? Huh?)
Paul Levitz on the other hand, is someone whose history rewrites are lame and unsatisfying, but at least are airtight and hard to argue with. For instance, remember the retcon that Validus was the deformed son of Lightning Lad sent back in time as Darkseid's curse on the Legion? While it was such a terrible idea it made me want to chuck that annual against the wall as hard as possible, I really can't argue with it intellectually:
1) Validus's powers are "Mental Lightning," a combination of Saturn Girl and Lightning Lad's powers;
2) Validus DOES have the mind of a child;
3) People from Lightning Lad's planet are in fact, commonly twins;
4) Validus's background was never truly explained, which means there's nothing for this story to contradict.
Whereas Byrne's DOOM PATROL rewrite made no sense, did not work with the framework of established DC History and is actually contradicted by it at several points (the Doom Patrol appearance in Superman and in JLA: YEAR ONE), and leaves various unexplained loose ends he couldn't be bothered with tying up (Changeling from the Teen Titans, for one).
So, all things considered this is more like Byrne's handiwork than Levitz's. Levitz was a fanboy turned writer, in the same category as Mark Gruenwald. Not BAD, but...you really wish Cary Bates had a few more Legion stories left in him.
If it WAS more Byrne's idea, then, well, there's just more proof on the whole "Byrne is really the devil" theory.
If it WAS more Levitz's idea, my respect for him falls from a sufficient, detail-obsessed but average writer, to one who is responsible for destruction instead of creation, the greatest sin in any creative field.