Your points are transferred after every statement to your SPG account, so there is no way to "loose" your points. Just make sure you cancel after the monthly accumulation has been transferred.

Did you check to see if you are eligible for the Amex Sync offer of a $60 rebate for $300 spend at SPG hotels? That should cover the better part of the fee right there. However, I really don't see the need for 2 SPG cards.