we are going to need the specific model numbers for the amps-- not just who makes them, so that we can check the rating numbers.
also, you said that you are connecting the speaker level inputs on one of the amps and line level inputs on the other amp--- this would also cause what you are describing.
speaker level inputs are high level inputs--- the amp is expecting a higher voltage on these inputs.
line level inputs are low level inputs---the amp is expecting a low voltage audio source on these inputs.
getting the 2 mixed up can cause one to be louder than the other.