Isn't that a problem worth solving? It had to be solved for banks to work as reliable institutions. Having a single blockchain in the end is probably as bad as having a single bank, although for slightly different reasons.
Its possible in the future we will develop a mechanism that allows two completely unrelated & unconnected blockchains to transfer tokens between eachother. I assume there would have to be either an intermediate chain specific to this purpose or both chains would need to follow a similar implementation. So far we started with Proof Of Work, we now have Proof of Stake and Proof of History. There may be more mechanisms in the future.
Is a cryptocurrency exchange more like a bank or a currency?
Atomic swaps already exist.
I still have no idea what that has to do with the nonsensical metaphor "a blockchain is a bank"