Re: Let's say Marvel and DC were to merge universes.
Well, there have been crossovers that show the two universes co-existing on the same Earth-- 2 Superman/Spider-Man team-ups, Batman/Hulk, X-Men/Teen Titans, Darkseid vs. Galactus. The second Superman/Spider-Man team-up, written by Jim Shooter, actually does have Clark going to work for JJJ and Peter going to work for Perry.
The JLA/Avengers team-up shows the two teams in separtate universes as does the Marvel vs.DC minis. The Marvel vs. DC minis actually briefly merged the two universes into the Amalgam Universe.
Whatever you celebrate this time of year-- be it Christmas, Hanukah, Ramadan, Kwanza, the Solstice, Life Day, Durin's Day, Festivus or whatever, let it be happy, joyful and safe.