The biggest technical problem here is that when two passengers appear together on a PNR, the segments must be identical — they must both be traveling on all the same flights, *in the same fare class*. This means that you cannot combine a coach award (fare class "X") and a paid coach ticket (fare class, say, "K") on the same reservation.

This is the same reason why it is sometimes cheaper to buy two separate tickets vs. one two-passenger ticket on a trip. If a flight's inventory is "Y9 B9 … T2 L1", you will be able to book a one-passenger itinerary in a cheap L fare and probably also book a one-passenger itinerary in a somewhat more expensive T fare, but you will *not* be able to book a two-passenger itinerary in L (the flight is only L1), and airline reservations systems can't handle giving you a single two-passenger reservation in "T for one and L for the other". It just doesn't work.

There is another technical problem with your proposal which is sort of unique to UA: UA is usually unwilling or unable to combine reservations which were originally booked separately. But even if they could, they wouldn't be able to merge two un-like fare classes together.

Hope your family enjoys the trip!