First off, many scientists will tell you that other universes aren't scientific to begin with because they can't be observed and are therefore completely speculative like Santa Clause or Nessie.
It might also depend on what you mean by universe. Our observable universe is 93 billion lightyears across. If someone were to stand 200 billion lightyears away from us, their observable universe would be a different one from ours, but it wouldn't be an overlapping reality separated by a magical veil as shown in the movies.
In terms of other universes as in completely unconnected in spacetime, the idea that gains most purchase in popculture and Chris's imagination is that if you assume infinite other universes, when we have no right to assume one, than anything no matter how unlikely MUST have happened in another universe. But even if there are infinite other universes they might be filled with infinite possibilities other than what you imagine.
If there were infinite other universes and you met your mirror image, this isn't a different "version" of you or a path not taken, it would be more like out of all the infinite universes there would be one where an Earth like planet also evolved creatures exactly like humans and one happens to look like you and even have the same name. But it wouldn't be another "you" in any meaningful way, it would just be someone who looks like you ala Prince and the Popper. Merging universes wouldn't cause people to become their counterparts, they would just be two different people who look the same.
Ideas like timelines diverging into alternate realities come from things like the Many worlds interpretation of quantum mechanics, in which observing a particle creates two universes, one in which it's a particle and one in which it's a wave. The many worlds interpretation is also controversial as other universes cannot be observed, and therefore it cannot be proven or even tested. Even if Many worlds interpretation were the case, it wouldn't mean there are alternate timeline versions of your life unless your life somehow hinged on one particle measurement. If many worlds interpretation is correct, then we might bore through the walls between universes only to find many worlds exactly like our own, but on one day in June 1967 a particle was measured as a wave instead.
The idea that other universes might have different laws of physics, and therefore allow science fiction and magic, also fails as we see in our own universe that slight tweaks to the laws of physics would prevent matter from ever forming. Some might even argue our universe is the exception to that commonality, so if you could visit other universes, you might have to wade through infinite voids and universes of nothing but gas clouds with no stars or planets until you find even one universe that has them.
Chris also fancies himself a goddess, creating these this other universe and changing its history at will. As established, another universe of your OCs would just be one that in a very long string of coincidences, happens to look exactly like what you imagined. It would be under no obligation or likelihood to continue looking exactly like what you imagine. And if you tried to change details, as Chris has changed the comics (such as turning gay vaccine into the vaccine for a plague) it wouldn't at all impact that universe, it would mean Chris started "chronicling" a different universe, and all the Sonichus and Rosechus from the first one would have to be abandoned.
And finally, these other universes wouldn't overlap our own divided by the "iron curtain" they would be dozens of billions if not infinite lightyears away, and therefore, inaccessible by our universe. Wormholes would have to be opened to connect you to another universe, but wormholes are their own mess with many reasons challenging their possibility, traversability, or the survivability of traversing one, and there would be no way for Chris to know whatuniverse he's accessing ahead of time. Quite simply, Chris couldn't go to other universes, and they couldn't come to us, we're all stuck here on Earth 1218