Dave are they fighting with the diathermies? (Dual/duel)
It depends on the make and type of machine whether there is a problem or not. Most modern machines have an isolated floating output so you can use two at once. I used to work in a cardiac hospital where they did it all the time harvesting a vein at one end and puting in as a by-pass graft at the other. But we chose our machines (Tyco Vally Lab Force-FX) with this in mind.
It is recommended that the plates for each machine be put near the area of use to ensure the current paths do not cross.
Robert