Well duh, that's already made clear by the mere fact that they can "jump" in and out of normal spacetime and travel at superluminal speeds.
But more to the point, as I was getting to a bit later, if another universe has a clear statement that speed of gravity is strictly "c" -- the two universes have been rendered inconsistant with one another.
Since when is the narrator of a novel automatically an omniscient, infallible observer? All you have to do is fine a single phrase anywhere in the entire book (or whole series of books) which is not perfectly semantically literally inerrant, and boom. No more infallibility.
So when reading 50's sci fi, do you say, "Well clearly the author was mistaken here and the black hole does NOT lead to another point in the universe. No, they ran across an entirely different phenomina that looks and acted like a black hole, but was most defintely not."
Personally, I tend to say, "Ok, so suppose black holes really WERE that way, even though we know that's not the case. Then what?" I would think that the infalliability of the narrator should be the absolute -last- thing to attack, and it should be something that falls apart because of an internal consistancy.
... in order for them to meet at all, they must occupy the same universe.
That's a defendable statement for any reasonable versus match. However, given the number of versus matches that involve different Earths -- or different armies from parallel timeframes, I'm not sure I see the problem with the idea of two different universes.
Actually, to state this better -- I don't see the problem with an infinite number of universes, two of which are almost identical save for a few minor, and currently unnoticable to your average 20th century scientist, differences, and these two then allowed to meet by some higher up technology that operates outside time and space. Stranger things have happened in sci fi.
Now that requires a bigger assumption than just trying to reconcile the two shows into the same universe, and for the most part, I'm all for the simple solution. But the original post did ask what -IF- the technology didn't carry over, and I still think that's a fair question.