Here is what the DVC site says about MM reservations (don’t pay attention to the dates, those are specific to that particular MM event, HS I think):
Party Size Limits
A Member can register themselves and up to 3 additional guests per Membership (for total of 4)—or up to the number of guests on his or her Resort reservation, provided it was finalized prior to July 13, 2020. Children younger than 3 years old do not need to be registered for this event.
Members With Multiple Resort Reservations
If Members have multiple Resort reservations, additional party size guidelines apply depending on how the reservation was booked:
When staying using Vacation Points or Reservation Points, Members can register themselves and up to the number of individuals on their Resort reservation(s).
When staying using cash or RCI® points, Members can register themselves and up to the number of individuals staying in the same room as the Member, per Membership.
**ETA - so it would seem that if you are booking both rooms with points, you could register everyone for the event.