Doing a 2-camera shoot using an XL-1s with MA-200 and an XL-1 with MA-100, lavalier mics are mixed on a Shure FP-33 mixer. Routing mic-level output from mixer to each camera results in screeching noise on headsets at each camera and at mixer. Disconnecting either camera from the mixer kills the noise and remaining camera has clean, mixed sound.
Why?

Are they AC or battery powered?
Are head sets connected to the camcorder or the mixer?

Try using isolation transformers in the audo connection to each camcorder.

if might also be that your mic's are possibly hearing themselves, thus creating some feedback - think of the sreeching PA system when it hears it self.

You could try to positition the camera's in arrangement so that the mics are not picking themselves up again.

Or is it a different kind of sreeching sound?

~Mike

Don, are you everywhere? Got your reply on videouniversity as well. Thanks.
I may have misled by using the term HEADSETS. We are monitoring with headphones only, not headsets equipped with mics. Noise is audible at each position and records on tape.
Cameras and mixer are battery operated. Cameras are minimum 10 feet away from mics.
Headphones are totally isolated...I can handhold a mic attached to either camera while standing next to the camera and wearing the headphones and not get the screeching. The ONLY time it occurs is when all three units are connected.