Whichever chain which has the majority hashrate (which will be the rest of the world) will be bitcoin. The other chain will be a fork of bitcoin.

If the Russians change nothing about their network and just continue to mine, they will fall well behind the rest of the network. AS ETFbitcoin has pointed out, mining in Russia would initially be incredibly difficult and the block time would be huge since they would be mining at the same difficulty with a small proportion of the global hashrate. They can do this, though, and simply generate a chain which is both shorter and has far less proof of work. If they choose to do this, then if there is ever communication between the two sides again (before any major new developments in the global bitcoin network) then their chain would lose to the main chain and simply be abandoned by all their nodes.

The other option is that they deliberate make their chain incompatible with the main chain (such as by forking to a new difficulty algorithm) which means if there was every a communication between the two sides in the future, their bitcoin fork would continue to exist as a fork of bitcoin.

Bitcoin would not exist twice. Either they fork to a different network, or they mine a chain split with the risk of their entire history disappearing at any time.