Clearing the desk and putting about 18-20" of space between the monitors fixes the problem. Who'd have thunk it?

Having the two screens 20" apart is more like monitor gapping than spanning, however, so I will need to live with the setup for a while and see if I adjust. If not, then perhaps I will experiment with an RF shield of some sort. Does a shield require grounding and would something as simple as sheet copper or heavy aluminum foil do the trick?