I know you didn't ask about Hertz but if you're an AAA member Hertz offers a second driver free if they also have an AAA card. Might not be worth it if the price difference is really high compared to Thrifty though.
Thrifty charges extra for a spouses. I think it's $8.00 per day. Alamo with the Wal-Mart customer code Didn't charge me for a spouse.
Wal-Mart Customers (i.e. anyone) can get up to 20% off rentals at Alamo. Use Rate Code BY and ID# 430710. MouseSavers.com reader Jennifer B points out that "the Wal-Mart codes for Alamo include a free additional driver."