You want to connect the two using a patch from somewhere on the front end of the amp, NOT a speaker out. This could be chained from the inputs if your amp has two inputs, could be a tuner send. You can run a line from a pre-amp out on the first amp to a power amp in on the second if both amps are equipped with this. Then you'd be using the controls on the first amp to control the sound of both amps, the 2nd amp would only be using it's power section (master volume). Effect send jacks work sometimes too, may require adjusting the input gain on the second amp. Headphone outs can be too hot to plug into the input on a second amp. Just make sure you don't plug a speaker out into anything other than a speaker. If you've got them connected now and don't hear anything funny or are having really loud volume with the volume knob set really low you're probably alright.