MargaretIllinois is correct. One way to do this (and it's probably the easiest way) is for one of you to transfer the needed number of points into the other's account. The two of you can call MS together to arrange the transfer - it's a relatively easy thing to do.
Be sure the points you want to transfer are current year points and that they will be valid for the time (and resort) for which you want to make the reservation. In fact, if I were you, I'd call MS ahead of time and ask their advice on exactly which points to use for what (reservation vs transfer) and when to make the transfer.
If you have to use banked or borrowed points, it will be a little more complicated. Keep in mind that you cannot transfer borrowed or banked points, but you can bank or borrow transferred points. Don't wait until the 11 month window opens to transfer points if you choose to go that route.
Another option is for each of you to reserve whole nights for the third studio and then have MS link the two reservations. One would reserve 4 nights and the other 3 nights. You cannot use points from more than one account to reserve a single night.
Best wishes -