It sounds like they need to veg out and want to spend quality time together. They are already committed to their family, and barely have time for each other.
Asking them about becoming mentors for Partners-in-Torah is asking them to take the little energy that they have left before they go to bed and devote it to strangers. While it’s noble in theory, imho it sounds counterproductive for their current needs.
Sorry I could definitely be wrong as well, and your idea may work for them. Either way, you brought up a good point of having them bring something new into their discussions together. That’s why I thought that they could learn something new together.