I've only read the Medium - have not had one played either as PC or NPC, but here is how I would run it as a GM:
A medium has to hold a seance, only one medium runs a seance even if there are more than one mediums at the seance. So, to even begin this possibility of having two seance boons, the attending PC's must attend two seances in the appropriate areas. If that actually happens I would say boon that affect the same thing do not stack. So, two seances calling a spirit champion do not grant +4 to all non spell damage, just +2.
BUT, if one seance called a Champion spirit and another called Trickster spirit BOTH adjustments would run, +2 non spell damage and +1 in one skill and that skill would be in-class to you.
So, NO - seance bonus does not stack, but YES - you can have more than one bonus from more than once seance.
