If you don't have a power conditioner, I'd recommend using at least a high quality surge protector. But yes, both amps and all effects should be running off the same outlet, ideally the same conditioner. Most ground loop problems are caused simply because amps are plugged into different outlets.
The back of the Amp Selector looks like this:
The ground lifts are necessary, regardless of whether you're using 2, 3, or 4 amplifiers. I won't go into the details of 3 or 4.
But if you're using 2 amplifiers, you would keep three of the switches in the down position, but flip either the 1 or 2 ground switch into the up position (lifting the ground for one of your amps) and, if you have your amps on, magically hear the hum coming through both your amplifiers instantly subside to absolute silence. And then many wide smiles will be yours as you unleash the tonal possibilities of blending two amplifiers. When running two amps, my favorite thing to do is use a pan pedal - have my Recto on a raunchy modern mode and my Crate on a clean, shimmery tone - then pan between them, giving me the option of going from balls out distortion to none at all smoothly, or staying somewhere in between for certain parts of songs. The effect is simply to die for.
Anyway, all of this information was given to me years ago, and I was not at all happy about it. I still, despite having been warned, tried basically every cheaper A/B in existence along with many different ways of eliminating ground loop hum. And I can tell you, this is the only thing that works and won't damage your amplifiers or totally wreck your tone. And not only will it not wreck it, it won't even touch it.
:!:
Just to warn you again, do not plug expensive amplifiers into three-to-two pin adapters. :!: I know this from hanging out in a house full of musicians who don't even have ground holes on their outlets, so they use these adapters on all of their equipment, and it's not pretty how much damage can be done. What's being said here about plugging amps into three-to-two's is a cheap way to fix the ground loop hum problems of having to plug your amps into two different outlets, when the reality is that you should invest in a power conditioner or, at the very least, a good surge protector and an amp selector.