Usually the discount code T&C will say that the recipient has to be one of the travelers, but ymmv if booking system will enforce it or just match on the last name.
Booking two one ways with everyone on the same reservations would be my choice, simply because if something happened to your flights and you needed to get rebooked, it would all be together.
If Customer Care isn't able to combine the codes or roll the $25 into one of the others, you might ask if they'll exchange it for miles and get some value out of it.