Your keyboard looks exactly like what I'm looking for, my Kinesis died. My IBM Model M from Unicomp is my favorite, but I need something more ergonomic.
There is one ergonomics issue that I hope you'll address. I build furniture. I built a chair with armrests to hold my computer. If the keyboard was split in two again, I'd run the cable under the chair, so they appear independent. If you drill 4 holes in each half, I can screw it directly to the armrests. Or just split the keyboard in two, I'll drill the holes down at the local hacker lab.
Your Mark 13 is a beautiful work of art. It is what I want in all ways except one; being split.
How about this as an option: Have a bottom plate that is all one piece, and screw the two pieces to it. Then it is identical to your current model. Someone like me could unscrew the bottom plate, and screw the top plates down where-ever, in whichever angle or orientation is best. I would screw each plate to the appropriate armrest and run the connecting cable underneath the seat.
Having my hands placed so close together isn't good for my neck or shoulders, which tend to get tightly compressed by regular typing. Opening up is good for the chest and physique. Having a split keyboard allows this. Yes, I could buy the ErgoMagic. But I like yours better. As for Kinesis, phooey! All that money, and it died so easily. Karma for what they did to Maltron?
The two halves would need to accomodate at least a 4 foot cable between them, longer is ok.
This idea also would be excellent for the standing desk I'm building, the "arm rests" will stick out from the wall.