The scenario of having multi EOS networks (2 or more) is not prevented. But it is assured that to happen and stay correspond to EOS rules, it has to be decided by at least 15% of the EOS tokens owners. The EOS token owners who owned the tokens at the time of the snapshot taken after EOS ICO had finished before any EOS main-net was every lunched.
So, to happen, it has to be the decision of the EOS early community (or a 15% of them at least).
If that happen, we will have more than EOS main-net simply like BitCoin & BitCoin Cash and Ethereum & Ethereum Classic. All run in parallel. Where, each one of them will have its possibly different running DApps, different Block Producers and even could have different governance. They in the extreme case, could be like Google Cloud vs Amazon Cloud. In which, each network has totally different machines, applications and operations.
Additionally, there could be for example, an entity that modifies EOSIO source code which is distributed under MIT Licence. They can for example change the adaptation process to make a new EOS network that does not depends on the voting of the 15% of the EOS early community.
At the end, having more than one main-net has its pros and cons. One of the good things is that if there will be more than one EOS network, this will be more attractive to Developers and Business owners. Because, using the same technology stack, they will have more than one option to use to deploy and run their DApps (Decentralized Apps) on.
Those different alternative networks could have different features. Such that, one of them could be for example cheaper while the other one is more popular.