Other brands use frequencies that are close enough that interference will be a problem. Just move farther away from each other and still have fun. You also have to keep one other thing in perspective. I don't think it would really bother your metal detector a couple of miles away, but 100,000 watts of anything is a lot of power. You need to look around you when problems occur and not automatically blame everything on the machine in your hand.
Metal detectors are very sensitive machines that are many times misunderstood.
But as we grow in our hobby and understand them better, we can better deal with whatever bumps lie in the road ahead.