18.2 Giving Mark-Room
(a) When boats are overlapped the outside boat shall give the
inside boat mark-room, unless rule 18.2(b) applies.
(b) If boats are overlapped when the first of them reaches the zone,
the outside boat at that moment shall thereafter give the inside
boat mark-room. If a boat is clear ahead when she reaches the
zone, the boat clear astern at that moment shall thereafter give
her mark-room.
(c) When a boat is required to give mark-room by rule 18.2(b), she
shall continue to do so even if later an overlap is broken or a
new overlap begins. However, if either boat passes head to
wind or if the boat entitled to mark-room leaves the zone, rule
18.2(b) ceases to apply.
(d) If there is reasonable doubt that a boat obtained or broke an
overlap in time, it shall be presumed that she did not.


If I am reading the situation correctly, the starboard boat is required to give mark room under rule 18.2(b). Remember that when sailing greater than 90 degrees from the wind, overlap applies between boat on opposite tacks.

Remember also that overlap is decided by a line parallel to the transom, so at the angles that catamarans sail downwind, boats in a significant area can be considered overlapped.

The starboard, outside boat is required to give room in this case. It is important to remember, however, that mark room is defined as:

Mark-Room Room for a boat to sail to the mark, and then room to sail her proper course while at the mark. However, mark-room does not include
room to tack unless the boat is overlapped to windward and on the inside of
the boat required to give mark-room.

Therefore, when overlapped in the zone, the outside boat only has to give an inside boat room to sail to the mark, then proper course while AT the mark.