I found this today. I don't know if it works but I will know soon.
Here’s how to combine the Alamo discounts:
Go to the Alamo website. In the blue booking window on the left side, type in your pickup location, date and time information.
Click on Add Discount Code and enter an ID number in the correct space. (An ID# may already be in that space. You may want to try other ID#s because some give better discounts than others.) Type a coupon code (if available) in the Coupon Code space.
If you are a Alamo Insiders member, click on Are you an Alamo Insider? and enter your username and password in the provided spaces.
You should receive up to two discounts — the ID# and Coupon discounts — plus any benefits that come from the Alamo Insiders membership.
Most coupons will list a specific ID# to be used along with the coupon code, but dozens of readers report using their coupons with ID#s other than the ones specified. We have successfully combined discounts from different sources (such as a coupon code from an airline frequent flyer program and an ID# from AAA) dozens of times without any problems.
Be aware that codes are subject to capacity controls. If too many other people have already reserved cars during the time you’re traveling, you may not be able to use your codes.