The only way to use two coupons is to have two separate rentals. The tradeoff is the convenience vs cost. If you get 500 miles per rental, two separate rentals also yield more miles.

Travelers Advantage will give a free voucher for a 3 month trial period for $1. They charge something like a $60-$70 annual fee if you don't cancel within the trial, but you can also get the money back if you cancel after they bill your credit card.