First how did you connect the amps together?
Next, separate the amps, and try them unconnected to each other. Does the amp giving you the problem STILL have that problem? If not, then there is something wrong with the way in which you have interconnected the amps. If the amp now has a problem when separated from the other amp, then likely something has broken in the amp. The only real way to tell it to describe the symptoms of the problem amp as it stands on its own unconnected to the good amp.
Finally, there are a lot of details the we can't determine without being there. Did you connect them correctly? Have you selected the right inputs? Have you set the volume to a balance sound level setting?
So, amps are very touchy as to how they interact. For example, I have a Yamaha amp pre-amped and driving a smaller Onkyo amp to drive a second set of speakers. If I turn the Yamaha (master) on first then turn the Onkyo (slave), the Yamaha shuts down. However, if I turn the Onkyo on first, then turn the Yamaha on, everything works fine. Just an illustration, of how complex the interactions can be.
The first thing to do is separate the equipment, and try the problem amp in the most basic configuration. This should tell you if the amp is faulty or if there is something wrong with you interconnects. It is possible that you have connected Input to Input, or Output to Output, or that you have some unexpected ground-loop problem.
There is a limit to what we can say without having eyes on the situation.
Steve/bluewizard