fyi, MS cannot link reservations made from seperate members or from multiple membership numbers under a single member (as i found upon booking 6 nights for a friend, who added 2 nights using her points). you can ask at check-in however.
you can rent individual nights from seperate members - it's just a bit more time-consuming. the other option is to have member B transfer the amt of points you need to member A, and member A would make the full reservation. most members would be reluctant to do this however, as there is a "one transfer in or out per year" rule.
you can also try david's rentals, a sponser on the boards.
hth