No ones mentioned the Dygma Raise, yet. I HIGHLY recommend them. It has a great solid metal build that doesn't flex. It's very compact. And it has hot-swappable switches which are very easy to replace. That was critical. I put in Zilents v2 67g switches and it feels great; very tactile and buttery smooth. I tried speed switches and Cherry browns too, but the zilents were the best for coding.
It's marketed as a gaming keyboard, but it's fantastic as an ergo. It's the best keyboard I've ever had. Previously I used the Microsoft ergo, forever.
Another subtle consideration is the placement of the Alt/Option keys next to the space bar. A deal breaker with a lot of other split keyboards, like the Kinesis, is that they have space bars that are too wide, so the modifier keys are too close to the outside edges. But the Raise has a compact space bar, so your thumb doesn't need to contort as much to use them. The software to reprogram the layers is decent, somewhat obtuse, but easy enough for basic remappings. For example, I reprogrammed some letter keys to be arrow keys when pressed with a modifier.
Oh, the only downside so far is that it really could use a row of function keys on the top, to mimic the macbook laptop layout.
They haven't released a keyboard tilt/tenting kit yet, so I had to make my own to give the inner edges a height of about 1.5 inches. Also, I place an elevated magic trackpad right in between the two halves. It's a frankenstein keyboard-mouse setup, but now it's very comfortable.