What happens when singer #2 is excluded from the mix? does the problem disappear?
He might be needing some mic technique training,
excessive use of vocal effects will contribute to feedback too.
I've had to mute his channel if his gain is too high. Not bad when his gain is set at the minimum. He just needs to get his filthy sewer closer to the mic!
I spent numerous hours last night and today reading pro audio "how to" guides and watching the same on YT. I'm convinced our issue is because of;
1) no gain staging / ringing the monitors out
2) poor monitor placement
3) mics are too hot and picking up the hard hitting drummer
and find one who'll use an electric kit 