If you mean for the same room, then to make a single reservation using both memberships would require one
DVC owner to transfer points to the other DVC owner who would then use both sets of points to make the single reservation. Transfer is a final transaction and thus if, for any reason the trip were cancelled, the transferred points could not be returned to the original owner.
A second method is to actually make two separate reservations with one owner using his own points to reserve the beginning portion of the trip in a room and the other owner the ending portion of the trip in the same type of room. The members would put on each reservation, as guests in the rooms, all the persons of both families so that the occupants listed for both reservations are the same. Then the members would jointly contact DVC member services and have DVC "link' the reservations. That will result in being able to have the same room for both reservations (as some have noted, that is not guaranteed 100%, but I have linked reservations between two memberships more than a dozen times and never had to change rooms).
Note that points from one DVC resort cannot be used for any reservation at another DVC resort until 7 months out, e.g., if you use the transfer method, and the points are from different resorts, they cannot be used together to make a single reservation until 7-months out.