You'll have to call MS as you cannot combine points from different memberships to book a night. MS will transfer points from one membership to your other membership and book the night for you. Since you have different memberships you won't be able to see the transferred points online.
Rules say that you may transfer points once per use year per membership, either into your account or out of it, but not both in the same year. If you transfer between your memberships, you will use up the entitlement for both for the use years in question. MS has been known to ignore that "one-transfer" rule when you are making a reservation for yourself, but you can't count on that.