Grounding, Grounding, Grounding!
You probably have a ground-loop somehere running between your Amps. Make sure that both Amps are grounded at the same point.
You might also want to make sure that any speaker connectors are not inadvertently touching the case of the amplifiers
.. I had that problem... In my case, it turned out that the fancy-schmancy turbo style gold plated RCA connectors were just a little big, and they were coming into contact with the amplifer case...
Hope this helps!
Glenn.