Yes, you can do that. Just keep in mind that you will get a JetBlue Travel Bank credit good for future travel for the amount of the flight you cancel as opposed to a refund back to your card. If you have enough True Blue points to cover both flights, that's a bit more optimal since the points would just be deposited back into your account.

The other thing to mention is that JetBlue often has schedule changes, and as soon as a flight has a significant schedule change that ticket is then eligible for a refund. There is little incentive to cancel much before a day or two before the flight as a schedule change (or cancellation) will get you a refund back to your card whereas canceling earlier guarantees you credit only.