Nice. That's a great way to eliminate the underage if one is over 25.
Nope. In that case the underage spouse is not allowed to drive. No matter what the agent at the desk says. I rented with Avis once and agent said spouses could drive no matter what. I read through the rental agreement and it said clearly that spouses are only added on as an additional driver if they are over 25. Under 25 they have to be added on and will incure underage fees.
If this is different by other rental companies let me know.