Gday McGregor,
From what I have been told you should be at least 25m from any other person with a detector to eliminate any interference between the 2 detectors. Im pretty sure that I may have read this on the Minelab site.
I have no friends to go detecting with so I have no practical experience of detecting in company. I do have friends but they would rather help prop the bar up at the local pub than get out into the great outdoors
Back in my uxo hunting days in Laos we were able to use the F1A4'S as close as about 4 metres from each other. We just kept working until we got interference then widened the gap.