Yeah, I figured I'd need to do some math
(value of the miles, the earned cost of checking bags, and other ways to get miles/how much it would cost to get that same number of miles otherwise...). I figured I'd just check first to see if there was some way to effectively attach the two bookings together and enjoy the bennies...but as I suspected, there's only so much magic one can manage in this case.
The flights are not terribly long, and we will all be in coach, so the number of miles in play here is not great. It probably makes more sense to pair up one of us with a card, along with one of them who doesn't (so one person on the paid booking has the card, and one person on the award booking has the card, that way each booking has one person who qualifies for the free bag which I assume is extended to both passengers on each booking, so we get all 4 bags checked for free, at the cost of foregoing just a relatively small number of miles. I can live with that.
Thanks for confirming and for the input.