Detector operating frequency and interference.
They can.
My dad and I both had Bounty Hunter Tracker IV for years. We hunted together but had to stay about 10-15 feet apart or they would scream at each other.
Some detectors, like my new X-Terra 505, have noise cancellation features, manual or auto, that will adjust the operating frequency slightly to find the quietest channel. This minimizes interference from electromagnetic sources like power lines and other detectors.
If your detector doesn't have noise cancellation and you're thinking about buying a second detector that will operate close by at the same time, consider buying either a second detector of a different brand/model that operates on a slightly different frequency or better yet, buy yourself an upgrade that has noise cancellation and let your friend use your older detector.