In an infinite Universe there will be mirror duplicate universes that could, just possibly, be traveled to. What most thinkers miss about travel between them (think what you might see in a hall of mirrors), is that the traveler will leave every single one of them at exactly the same time to travel to and arrive in some other mirror duplicated universe. All will leave at once out of the same frame duplicated to infinity in the mirror, all will arrive at once into the same frame duplicated to infinity. Thus, just one and one one only of us.
Terminology:
What you are talking about is
observable universes. In a universe of infinite extent (of which, by definition, there is only one), there will be and infinite number of observable universes.
Combinations and Permutations:
You are missing the point of an infinity of universes.
For every universe out there that is identical to ours, there are uncountably more that are
almost exactly the same but not quite - one atom out of place, etc. There's even more universe out there where 100 atoms are out of place. etc.
There are an uncountable number of observable universes where I flew off in my starship ten seconds later and arrived in the next universe ten seconds later.
There are an uncountable number of observable universes where I flew off in my starship ten
minutes later and arrived in the next universe ten minutes later. etc.
There are an uncountable number of observable universes where I flew off in my starship ten
years later and arrived in the next universe ten years later. etc.
It misses the point to concentrate on only the ones that happen to coincide.
There is no time travel without space travel.
Muons experience time dilation during a trip from the top of the atmosphere and the bottom of the atmosphere. Does that count as "space travel"?
No space travel without time travel.
There is also no "sitting stationary not going anywhere" without time travel either. Because we are always travelling through time. So it;s kind of a truism.
It really is spacetime travel. If superman with his supervision standing on the Moon spots a piano falling from a building a few floors up about to crash and crush a cat walking on the sidewalk, and he (superman) can cross the distance at infinite speed, he will still find the cat crushed by the piano and dead because the observation itself of the accident about to happen was far too late in getting to him.
This is not necessarily true. The light from the falling piano takes a little more than one second to reach him. If it takes the piano 2 seconds to fall then he will arrive in plenty of time to save the cat.
Even at infinite speeds he must travel concurrently existing future light-time histories across space rather than past histories which will always be only one photo-frame flat (only one photo-frame deep), the one in his eye.
This is word salad. You are making a mountain out of a molehill.
Superman sees the piano falling after a 1.25 second delay. If he flies home at "infinite speed" (which he can't, but whatever) he will arrive as soon as the piano started falling. It takes a finite length of time to fall. If that time is longer than 1.25 seconds, he will arrive in time to save the cat.