Hey! I've run two-amp rigs in a bunch of different bands and might be able to help here. One thing though, to avoid confusion: "bi-amping" specifically means using two amps to cover different parts of the frequency range, usually so you can put more power towards the low end. Usually a bass (think GK800RB) or PA thing, not guitar.
Anyhow! A few things to keep in mind...
- if you want to try this out, just plug the amps into the two outputs of a regular Boss tuner. Alternately, if one of your amps has two inputs (that aren't going to separate channels, i.e. don't have any separate controls) you can just plug your guitar into one of them and connect the second input to your other amp
- if you eventually get an A/B/Y pedal, there are two kinds: buffered and un-buffered. Basically, if it works without a battery or power adaptor: it's unbuffered. Some unbuffered pedals will still have LEDs, which can be deceptive
- I've found that using multiple amps without a buffer noticeably alters the way my amps sound. Your mileage might vary depending on your setup though!
- frustratingly, some amps are out of phase with each other and will sound thin and weird together. Some ABY pedals have a phase flip switch on one output (very useful!) but if that's not an option, you can easily build a phase-reversing speaker cable by swapping where the two wires attach to the plug at one end.
- if you find that you're getting extra hum when both amps are connected, you probably have a ground loop. Supposedly the "ground lift" switch on your amps, and possibly on your ABY pedal, will help with this but in my experience they don't. Get one of those little 3-prong female/2-prong male power plug adaptors and use it on one of the amps.
- I've never seen a footswitch that is built to connect to two amps at once, and I don't know whether the Mark 4 and Tremoverb use the same kind of switch in any case. I've had good luck building and jury-rigging switches for Mark 3 amps, but I suspect the Mark 4 is more complicated.
