If black holes are universes, then what happens when 2 black holes merge? What happens when universes merge? The sudden appearance of more stuff? More space? Existential cataclysm?
It's simply the plural of "universe". If there actually is more than one universe, it would imply that "universe" was a misnomer, much like "atom" turned out to be.
People might keep using the word with a new definition. The new definition could be something like, "a region of the multiverse that is causally disconnected from other regions of the multiverse under such-and-such conditions." If those conditions change, two universes could become causally connected and merge into one universe. I imagine inhabitants of those universes would (if they look in the right place) witness events with no recognizable cause until the merger was complete.
Or what we currently call the universe might be renamed a sub-universe within the actual "one" universe, which was much bigger than we thought. Then the question is what would happen when sub-universes merge. Same question, different glyphs.
With our current theories such questions are nigh intractable.