I often have choirs accompany the a cappella band I mix. I put a powered speaker on a stick in each wing as side fills and route only the bands signal, not the choir mics, through them. Usually just my beatbox and the lead for whichever numbers we are performing. I push the monitors to such a level that when I ask the choir, collectively, if the volume is acceptable, I look for a solid majority to give me head nods or thumbs up. If I can hear the side fills in my choir mics at that point I duck them significantly before the next soundcheck moment and pray for the blind to lead the blind. If most of the folks on the edges of the choir riders have a good idea of where they are in the tune, the rest in the middle will typically follow their lead.