Easy. You'd probably pick the one with the biggest market cap. The whole point of money is that it is useful because everyone else is using it. So the one with the biggest market cap is almost certainly the one that you'd want to follow.

Or you could not do that. You could instead pick a different one. There is nothing stopping you.

If there are multiple networks, it would be obvious, and you can merely make up your own mind about which to follow.

Interestingly enough, this also applies to POW chains. There is nothing stopping you from picking a shorter chain. But if I had to guess, I would say that almost nobody would be following the shorter one.

This applies to POS coins as well. There would likely be 1 chain, that everyone is following. And this one chain would be obvious.