Why ? Because both companies have their fair share of alternate universes, there is a possibility that both universes exist in each others alt. universes. And there have been crossovers such as superman vs spiderman and batman vs the hulk. And marvel has a specific earth for this, they call it the "crossoververse": http://marvel.wikia.com/Earth-7642 . It's actually more likely that dc exists in marvel than marvel exists in dc. Because marvel has millions of alternate earths while dc only has 52.
Do marvel and dc coexist
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