Board Thread:Questions and Answers/@comment-27984457-20160718190207/@comment-4651179-20160718204216

No, a fusion would consist of the threads of space and time that form each of the two realities merging into a single one.

What happened with the current Marvel Universe is that after being brought back, several inhabitants from different destroyed universes found their way into it.

To continue with the analogy, the doughs of two pizzas weren't mixed together to create a new merged pizza, the toppings of one pizza were added to the other pizza without changing it fundamental characteristics.