CW, welcome back. The M60v3 can have two sets of binding posts installed for a $40 premium(I had suggested that the premium be $1000). This is an essentially useless provision that could allow meaningless bi-wiring or bi-amping which isn't actually done in any significant numbers of home setups.
As to your present setup, if you've described it completely(and if we've understood what you've written), this is a bad practice, as Rick pointed out, and should be discontinued regardless of how long it's been in effect. The path of the power(voltage)flow would be from one amplifier down its wire and into the M60 and its connecting strip(which you surely have connected). Then, since the voltage doesn't know that you'd like it to go only into the speaker, it also goes from the connecting strip up the wire into the output stage of the other amplifier(regardless if it's off). The result should be at least increased distortion if not actual damage.
