Booking Alamo car rental via Costco, second drivers are free. They do not have to be a spouse or other family member or even a Costco member. From the Costco web site FAQs (
https://www.costcotravel.com/Info/blog-rental-cars):
"4. Do I get an additional driver for free with my rental car reservation?
Yes. Costco Travel rental car reservations include one free additional driver at U.S. locations with Alamo, Avis, Budget, and Enterprise. At Canada locations, reservations include one free additional driver with Alamo and Enterprise only.
The primary driver must be a current Costco member and be prepared to present a valid Costco membership card at the rental counter. Additional drivers do not need to be Costco members, but must be present at the rental counter and provide a valid driver’s license to be added to the reservation. Additional fees will apply for drivers under 25 years old."
Note 1: the Costco member has to be present when picking up the car, prepared to present a valid Costco membership card. In practice, I have never been asked for my card.
Note 2: I always do online check-in and skip the counter (when available), so I can go directly to the rental car pickup area and choose a car. The online check-in form includes the opportunity to add the second driver (with driver's license information). When checking out, I simply hand the agent a copy of my online check-in/skip the counter confirmation, credit card, and the two driver's licenses. Never had a problem with this at MCO. In San Diego, I was able to check-in online, but had to use a kiosk to complete the process, since skip the counter was not available at that location. I had to scan both driver's licenses as part of this process.