Hooking up two fax machines to one line is the same as hooking multiple extensions in your house, but the phone company would like to limit the number of extensions to 4 because it hinders the ring voltage (the more ringers, the lower the voltage going to each). If only one line is available (no PBX) both machines will ring at the same time and probably pick up if the ring option is identical. If not, the one with the less ring will pick up first and of course, the next call will get a busy while the machine is processing the call. Keep in mind that if one fax machine goes bad, it may affect the line so that the calls will either get busy or ring-no answer. To check, just disconnect the suspected fax. If you have a PBX, then the hunt uption works best. Good luck