It is a new coin. It's not pre-mined. The Bitcoin UTXO set will be copied, so everyone that holds BTC will receive B2X, much like a dividend to a stock holder. A hard fork of a blockchain creates a new currency out of thin air. Before the fork, there will be a circulating supply of about 16.6 million BTC. After the fork, there will be a circulating supply of 16.6 million BTC and 16.6 million B2X.

So, if we accept B2X as "Bitcoin" then we are effectively doubling the supposedly limited supply of bitcoins. This is one reason why a permanent split with disagreement over what constitutes "Bitcoin" is such a problem.