I think the crossing return paths problem is to do with localised heating.
As the retun currents pass through the body there will be some IsquaredR heating happening. If these two paths cross, at that point there will be two lots of this heating so more likelyhood of tissue damage.
Using two similar diathermies is the best option. If you use an old one that does not have a floating output the REM monitor (or equivalent) on the new one gets very upset.
Robert
