
Solution. Get a figure 8 mic and split the singers. Half singing in the front and half into the back.
Then set up an old set of speakers equidistant pointing at the mic side nulls.
You will hardly be able to hear the spill from the speaker monitor mix as it is pointing into the null and as the speaker waveforms are out of phase they will cancel at the mic.
You can use this for people that just hate headphones.