They will not merge, but transactions will homogeneize.

I have simulated this scenario, forking the Bitcoin chain on a private network 2 years ago. What happens is that if the chain is forked, and if a client unexpectedly sees that another chain (say chain B) suddenly replaces the chain it was following (say chain A), thereby reverting a bunch of transactions, then the client will automatically retransmits its transactions that it had saved in chain A, to have them saved again in chain B.

That is how a non-malicious client operates.

Of course, if you are a malicious person, you can actually kill the client and manually wipe out its cache of transactions before it has time to retransmit transactions for chain B (therefore recovering coins that you had spent on chain A).