powerful villian of either universe escape his followers threw a dimensional portal.
With him is a force of Metahumans or soldiers equiped with hightech weaponry.
By accident they end up in the other universe the portal beeing wrong. The superheroes that follows find the dimensional portal and decide to go threw.
A Dimensional Rift have come to existence wrongly by nature. The rift is very hard to discover. Characters become miss placed and maybe the problem attract the attention of superheroes from bouth universes.
The Rift may also be growing and what will happen then ? Maybe there will be one universe or they will destroy each other, the rift have to be closed.