I've had a Special 6 for a while, and have done a few mods to it. I love the tone but am only using it for practice. My gigging amp is a Kendrick Bassman, which I also love. But I would like something in-between, when I'm in a smaller venue. Small enough not to have to lug the Bassman, but too large for the VHT by itself. My band does not mic amps unless we are working with a sound person, and that's rare. So I need stage volume.
I've looked at the Princetons, and some of the 60's Gibson amps. Too expensive, too old, too fragile, etc. And I love the tone of my VHT so much, I would gladly use it if it were about double the wattage.
So why not double the wattage with a 2nd Special 6? OK, now the question: how? Plug into the Hi input and out of the Low input and into the Hi of a 2nd VHT? Use a splitter to go to both at the same time?
I know someone is gonna say, "just mic the amp" or "just add a line out." I know I can do that so it won't be useful to me to hear that here. But without someone running sound for us, it's impractical and the mix is too difficult to monitor from onstage.
Speaking from direct experience with small amps and such...
Spend the money on a good A/B pedal like the Radial Twin Cities. Get the best you can buy. The cheap route is a basic Y cable, but you lose signal, can get hum, might get shocked, etc...all things I've dealt with first hand.
Secondly, you would be creating a bigger sound doing that for sure. You'll have some sweet bass coupling going on. Will it be louder? Maybe just a hair...BUT, it won't have more cut or travel further than a large amp.
You'd be better off with a larger Push-Pull amp like a Princeton Reverb Reissue. The two amps won't add much for monitoring, and they won't carry a room.
I used two SF Champs in said manner and it was a huge headache compared to bringing one larger amp. If a different amp isn't going to do it for you, I would do the following...
Mic the amp and send it to the internal mixer of a powered speaker (awesome ones can be had for under $500 all day long)...take the lineout from the speaker/monitor and send it FOH.