In 1996, Marvel Comics and DC Comics co-created a Limited Series called "Marvel Vs. DC":

Where it was revealed that the multiverse of each company's main continuity was in fact the creation of two Gods simply known as "The Brothers".

For eons, they were ignorant of each other until the first crossover brought them to each others' attention. Each wanting to be the singular God of the Multiverse, they decided that the matter could only be settled with a series of one-on-one battles between the most similar heroes from their own dimensions, the stakes being that the loser's universe would be destroyed, giving both sides had plenty of motivation to kill each other.

At the same time however, a young man named Axel Asher, A.K.A. Access, discovered that he was a shard of the original joined reality. This was apparently a mantle that was passed down from generations, Access could only stand by and watch as the two universes did battle. The Marvel universe was ahead six to five, when suddenly...

...the Brothers decided to settle things their own way instead of waiting for the battles to finish. So they seemed to merge, creating a single universe out of the two: the Amalgam universe.