It is correct that bitcoins mined before the fork will be spendable on both chains. That being said, it is also possible (and in my opinion probable) that the combined value of both currencies (BTC and BXT) will be less than the current value of BTC for a variety of reasons (Metcalfe's law, loss of confidence, etc.). That being said, I think it's more likely that one of the forks will win (e.g. the core devs will merge Bitcoin XT to Bitcoin Core if the fork happens).
