Apparently, it's OK, because commercial boats have two or more anyway.
It probably relies on random variations in the crystals. If you had two transmitting on exactly the same frequency, from the same spot, I don't see how you could modulate any information (e.g. the GPS position) on to the carrier. The data rate must be low enough to keep the sidebands close enough to the carrier, so that two of them don't interfere.