In the comics, they did a quasi-sequel to Crisis called Zero Hour: A Crisis in Time. The idea was that this time various time lines were being destroyed (which is not a firm distinction from parallel worlds, in my opinion); and as time collapsed, the various histories began to overlap.
The comics visually represented this by having each character presented in their original art style from that period:
This is kind of what’s happening with the actors in the Arrowverse shows. The comics didn’t try to explain why the Batmen looked different; they just put it out there, let those different personalities interact and fans just took it as a neat little nod to the history they knew or a pointer to check out things they hadn’t seen before.
But from a dimensional perspective as is currently theorized, we perceive reality different depending on what dimension we’re looking at it through (dimensions still denoted as a way of measurement and not a metaphysical concept as many often take it).
We live in the 3rd dimension of solid objects with length, width and height. We master that.
We can perceive the 4th dimension of time but only in a limited, one way manner. Travel or perspectives from the 4th dimension are Avengers: Endgame logic. You can change whatever you want in the past, but the future you return to will always be the one you left. You’re on one track - if you want to see that alternate future, you’re taking the long way.
The 5th dimension is alternate timelines. This is Back to the Future. You change something in the past; and when you return to the future, you’re now on the new timeline and don’t get to go back to the old timeline.
The 6th dimension is parallel probabilities. For this, you don’t need time travel or any effort to make alternate time lines; you simply walk from one to the other. These alternate realities will be things that look familiar.
The 7th dimension moves us into parallel possibilities. The laws of physics begin to break down from what we understand. You start seeing things that don’t make as much sense to you, but there is still some sense of things you do understand. This begins to move us into areas of magic and folklore the further into the 7th dimension you go.
The progression on Sliders was showing us just how lost they became as time passed. Like someone walking away from their house, they are at first going to see things and landmarks they recognize. But the further out they go, they are exposed to things they never expected or even believed existed.
I believe what caused the Sliders to become so startlingly lost in season three started with Logan swapping out parts on the timer for less reliable technology (Double Cross was meant to be the first episode of season three). The Sliders were pushed further toward the edges of the 6th dimension dipping into the 7th when they swapped to the Egyptian timer and it’s technology. The Sliders were pushed even further toward the 7th dimension when they began to follow the path blazed by the more powerful Rickman timer.
So this is where I would put the actors playing dual roles in the Arrowverse Crisis. Characters like the Brandon Routh Superman are coming from the far side of the multiverse where realities start to bleed into the 7th dimension where things start to not make sense. As the antimatter wave progresses consuming the multiverse, it’s going to flush out these oddball refugees of alternate realities that are retreating toward Earth 1 (or at least that’s how I would handle it). But these characters will be those that the Arrowverse characters would have never interacted with but for the collapse of the multiverse forcing it - the characters were simply too far apart until the multiverse began to “shrink”.
Incidentally, by our modern theories Mxyzptlk would most likely be from the 10th dimension. However, he was established as being from the 5th dimension so many decades ago that they are unlikely to ever change it.
