They never merge. But each should want to (and be able to) subsume all non-conflicting transactions from the sibling chain, so it's only a small number of 'double-spends' that wouldn't live in both. Eventually one history will win, and only the conflicting transactions from the losing history lose out.

Unless...

Perhaps an enduring, permanent split, under the right conditions, with people motivated to keep both chains alive indefinitely, could be a good thing. Among other things, it might solve the deflation problem, or allow a runt network to continue if the mother network were compromised by technical or legal attacks. (Or depending on your perspective, allow the mother network to continue if a runt network is created by certain attacks.)