Dude...told you the way to go - Little Labs PCP Splitter. This is one awesome piece of kit and I wouldn't be without it. I am running 3 amps (Vh4, Uber, JCM800) in/out and not one bit of hum. Everything can be ground lifted and phase altered independently and impedance matched. I haven't used the direct in/outs yet, but they are on the box. Jonathon makes rack ears for it, which I got, so it stays in my amp rack and wired up.
I too was looking for a Systematics System Splitter but couldn't find it. Googled a few things then Little Labs came up. Its a popular studio tool, but mine goes onstage too. It is pricey as hell ($1000), but for multiple amps and not having to deal with grounding and phasing issues, this is the only way to go. You don't have to muck around with those noise gate and hush things either.
It is also used as a reamping too, but that is beyond by needs. You can download the manual from the site. Email Jonathon and he will get back with you with answers. Got mine from Mercenary Audio (
http://www.mercenaryaudio.com). They carry loads of high end stuff.
http://www.littlelabs.com
If you happen to run up on Systematics, let me know. The only reference I found of them was in a Joe Barressi interview about the recording of Tool's 10000 days record.
Steve