BIP148 results in a soft fork of the legacy chain. This means that if it were to overtake the legacy chain, it would completely replace it and would become the one and only Bitcoin blockchain.
Upon activation, the choice will be between a chain that is safe from getting replaced that supports SegWit and a chain that can be completely obliterated that does not support SegWit. Many of us believe the markets will heavily favor the former, compelling more hashpower to move to the BIP148 chain until it becomes the dominant chain.
It is possible we get a chain split as a result, but I believe the chances of a permanent chain split are far lower from BIP148 than trying to force a contentious hard fork given the current circumstances — and I’m not convinced that Bitmain actually seeks to activate anything at all and suspect their shifting unreasonable demands are merely a stalling tactic. A contentious hard fork seems unreasonably risky for them and they’re making a lot of money with the status quo collecting fees from the rest of the ecosystem.