Can you have one seuda for both a bris and shabbos/yom tov seuda? Can you have one seuda for bris and siyum and yom tov seuda?
The halacha of ain mearvin simcha b’simcha only applies to making a chasuna, etc. during Yom Tov or Chol Hamoed, but not to making a bris seuda, or a siyum. Therefore, it would seem that even if it is for both the bris and a siyum it would be permitted.
Shulchan Aruch O:CH 546-4, M:B 11.