In what way is an alternate separate earth population functionally different from an alternate universe? You say you're trying to avoid a scifi scenario but your two proposals are already pretty silly scifi.
If open-endedness is a problem, simply limit your universes to 2, like in Hominids.
Also, it would be easier to give recommendations if I knew what argument you were trying to win with this thought experiment.
I'm not trying to win any arguments. I'm trying to reason about artefacts of human culture that are parochial (accidents of history) or human-universal (practically inevitable products of human history). More to the point, I'm trying to equip other people with tools to reason in a similar fashion.
I'm also not trying to avoid sci-fi scenarios, but I am trying to avoid scenarios which have such a long history as a sci-fi trope that they will inevitably influence people's intuitions.
I'm not writing a story (although I do want to frame the thought experiment...
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.