Exchanges and merchants transact... with users. Also, you definitely don't have to use the longest chain. That's one of the key innovations if Bitcoin, you only

Exchanges and merchants transact... with users. Also, you definitely don't have to use the longest chain. That's one of the key innovations if Bitcoin, you only follow the longest chain that your node sees as valid. At the moment of the split, most nodes will not see the 2x chain as valid, so they will not follow it even if it is the longest chain. But those nodes will keep working.