Yes, because while we know it's perfectly plausible, disregarding some small oversights in your scenario, such as the early inflationary expansion of the universe was not limited to the speed of light, however our universe was at that time constricted to a singularity all that existed was compacted into a space smaller than an atom. If the big bang theory is correct, and it's the best one to date, 42 billion light years away didn't exist.
Also, the universe is already larger than 42 billion light years in radius, so unless your theoretical universe began on the opposite side of the universe from earth, we'd have been impacted by its expansion already.

That assumes that space exists independently from the universe itself and that the universe is expanding into it. This is not the view of modern cosmology. The Big Bang made space itself. At least the dimensions that we know about. Other "universes" are certainly possible and even, in my view, likely. They do not exist in our space though they may at times affect it. So something like what you are considering is possible though the cosmology is more complex.

your argument is contradictory and you are confusing time with distance, please rephrase the question . you can not compare a time of 14 billion years with A Distance of 4.2 billion "light years"

No, but I'll never be able to prove it, since if I/we could see it they would not be separate. I think

I find belief that there is only one to be irrational.

There is a term multiverse.This is based on the theory of eternal inflation,which suggests that shortly after the Big Bang that formed the universe,space-time expanded at different rates in different places.According to the eternal inflation theory this gave rise to bubble universes that may function with their own law of physics.

The short answer is yes.

However, no knowledge is 100% certain. Every truth is based on a probability of it being correct. The higher the probability, the more likely it is true. The lower the probability, the more unlikely it is true. That is why we test and re-evaluate.

The descriptive part of your premise is not required. Much of quantum physics posits multiple universes.

It's a bit more complicated than that; if these two "universes" could see each other, then by definition, they would exist in and be part of the same universe. If multiple universes do exist, we cannot be aware of them through any conventional scientific observance. The possibility has been shown mathematically, but is only theory at this time.

Beyond our knowledge or our capability to determine.

Nope.... you see its what you think that matters.

I don't think so at all. In nature, the physical world, I don't think there is only one of anything. Multiple stars, planets, galaxies, etc... . Why would we assume there is only one universe? Because we don't know of any others? I don't know of a lot of things but that doesn't mean they don't exist.

Not according to theorists who talk about the "multiverse". Consider yourself in good company.