I have a bit of a fascination with the lamentations-of-the-flame-princess summon spell (Grindhouse Rules & Magic p142). I am wondering if there are any rules or guidelines for combining HD of summoners?
I would think summon is a case where combined power would be the norm, if a powerful creature is to be summoned by those foolish enough to try. This combination could be a coven of magic-users, or a group of cultists. Using The Devil Rides Out (Wheatley, 1934) as a bit of an example, Mocato (a powerful magic-user) is able to summon the Goat of Mendes, which would certainly be an arch-fiend. There is a mob of cultists, many of which are likely 0-level in game terms. There is very little in the way of sacrifice (possibly an animal, but I do not recall from the book or film). Mocato might be powerful enough to summon it solo, but would also be smart enough to stack the deck in his favor as much as possible for the inevitable Domination roll. Also of note, this summoning occurred on an unholy day.
Notes on sacrifice from p143 of Grindhouse Rules & Magic:
Each full 2HD of sacrifices gives the caster a +1 bonus to the Domination roll, or 1 HD for a +1 bonus if the sacrifice is the same race as the caster.
So, the question is are there LotFP rules concerning combining power in lieu of sacrifice, and from what source? I have looked through the Grindhouse Rules & Magic and the free Magic book, but may be searching for the wrong term. This question assumes that there is a single spell caster, who knows the summon spell. Perhaps there is something similar to "every 4HD of cultists or every two levels of magic-user participating in the summons (apart from the caster) provide +1 to the Domination roll"?