When I was 15 years old, I tried using two amplifiers at the same time. There was tons o' noise and it had a honky sound to it. In the years after my ill-fated attempt at a multi-amp setup, I learned a bit more about how two amps can be in phase or out of phase with each other when used at the same time. I'm sure you've heard these terms before, but if you haven't, or if you're a little sketchy on the subject, I’ll go over them quickly before giving you a few tricks to make it easier next time you want use a two-amp rig.
When an amp is in phase, the speakers will push—literally. I'm sure you've watched your speakers when you’re playing and seen them move back and forth. The first tone that is emitted through them will make the speaker basket push toward the grill cloth of the speaker cabinet. That is in phase. When the speaker is out of phase, the first tone will make it draw back toward the speaker's magnet.
The reason an amp pushes or pulls is that every gain stage your amp has will flip the phase of the signal 180 degrees. For example, the process might start in the 12AX7 tubes in the preamp section of your amplifier. Unlike a power tube, every 12AX7 has two sides, or triodes, to it. It's basically like having two tubes inside that little glass bottle. On a blackface Fender, the normal channel will use both sides of the 12AX7 in the V1 position (tubes are numbered V1, V2, etc, beginning closest to the input jack). This gives you two gain stages. On a plexi Marshall, the normal channel will use one side of V1 and both sides of V2 for a total of three gain stages, which of course is 180 degrees, or out of phase with the Fender. That's the basic idea, and it’s enough to cover what we're trying to achieve here.
In the past with Brad I've used up to six amps in the same rig, with anywhere from two to four on at the same time. Getting all of them in phase with each other in their various combinations is a trick to say the least. When amp 1 is in phase with amp 2, it might not be when used with amp 3, but 3 has to also be in phase when it's turned on with amp 4. Confusing? Yes, especially when any of those amps could be switched out from day to day. But, there's no need to break out the physics books here—we’ll just be discussing a quick reference, so you can tech your own rig if you decide you want to run multiple amps.
How you split your signal between the two amplifiers is up to you. Keeley's Framptone amp switcher works great. Axess Electronics’ BS-2 buffer/switcher is another good one. Both are a single input with dual outs, and have the ability to flip the phase on one of the outputs. There are tons of these on the market, so finding one is really easy. Of course, any stereo output pedal works as well.
I tend to check my speaker cabinets first, so a quick test can be done with a 9V battery and a speaker cable. Plug your cable into the cabinet’s input and touch the 9V to the cable's 1/4" end—ground to the sleeve and hot to the tip. You'll hear a pop and see the speakers move. Just like when a guitar is played through them, if the speaker cabinet is in phase, the speaker (or speakers) will push toward the grill cloth. This is what we want—all speakers moving the same direction all the time.
When you play your guitar between the two amps you may hear a hollow type of sound between them. This happens when the speakers are moving in opposites directions. It's almost like there's a hole there and the tone will have an excess amount of midrange to it. If you're using a pedal like the Framptone, you should be able to cure the problem with a flip of the phase switch. If you’re using a stereo effects pedal, you'll have to flip-flop the speaker wires coming out of one of the amps. Just switch the hot and the ground wires on the speaker terminals. If your amp has two speakers in it, you'll want to switch only the two wires that are coming directly from the amp. Remember, there are dangerous high voltages inside your amp, so if you do not feel comfortable, consult a technician.
Turning on certain compressor pedals will also flip the phase. Just like the amps, the compressor has a gain stage, so the signal will be turned 180 as it passes through. Not all compressors on the market today do this—some specifically state that they won’t change the phase. If you have one that does, try putting it on just the out-of-phase amp. It's kind of a cool effect to compress one amp in a two-amp rig to give a little more variance between them, so using one in this application yields two results—three if you count not having to rewire your cabinet.
One final note for using a two-amp rig: sometimes ground noise can be an issue when using several pieces of gear. A couple of ground lift adapters on hand should handle anything that might pop up. Take your time and lift each amp and your pedalboard, one at a time, until they're all quiet.