I'd recommend not using any cables other than USB for the connection. If you want one side to connect to the other hang a hub off the one with a port off the back for the other to plug into. Most controllers (all I've seen at least) stick a hub first thing in the chain anyhow. Anyhow non USB cables will come back to haunt us for sure (always did in my experience at least). About the VID/PID don't try and get custom, ain't gonna happen. There are some generic blocks we can co-op I believe (been a while since I messed with this).
Edit: Yeah, two controllers, one with a hub if you like (nice esthetic but I don't personally care) and input USB port or two (can hang the local mouse off the other if you like - Kinesis does this). No ribbons, that'll be messy - to solder and maintain. No other cables, needs emphasis, you'll be chasing down all sorts of nasty issues (reliability, specs, etc)
Edit-Edit: Caught KPS's post. "Doesn't work on OS X, which treats separate keyboards as fully independent — a modifier on one doesn't affect others." Right. Maybe USB-HOST? Darn, might have to rethink this, cables scare me (must be a bad childhood experience)
Missed this comment earlier from dorkvadar "Architect: you prefer the maltron thumb area?http://www.maltron.com/media/hi-res/..._tb_black1.jpg
I don't see how we can add that, but a 1x key out between the 2x thumbkeys and the normal keyboard keys is surely possible. Maybe that's what we're after?"
Yeah I'm good. I never realized that about the Maltron - never tried one either. Anyhow I love the design now, don't change a thing, just be careful with the cables (and I'm good for three, yikes ...)
Personal commitment, I'm 1-2P (1 with 2 more pretty sure, didn't realize how much DIY is involved (too busy to DIY these days)). Put me down for having somebody do the soldering, thanks.