TBH, I dunno either. I never played the game but its my understanding that they specifically made the buffs short duration to prevent pre-buffing (cant stack buffs on everyone because they will start running out before you even engage an enemy).
The D&D version of Sequencer, as implemented in BG2, did exactly that. It would hold 3 spells of level four and below. The point I was trying to make is that its functionally equivalent of three attacks at once, so I would be curious if the devs provide the same for buffs.