I supect the free month is tied to an account...so yes you could have 2 free months, but they'd be on different accounts. For example you could hypothetically start up a MidshipmanRob account, play it for 1 month till you worked out what you were doing, which faction/ship you were going to play etc, then start your CommodoreRob account with the other free month.
Thats just my pressumptions based on other online games I have encountered. A lot depends on how TAG's accounting database is constructed. They obviously aren't going to be complaining if people who bought retail buy gold too, but the crediting of the 2 free months to 1 account may be technically difficult to achieve.
/bux