If you want a smaller version of the Ergodox, you could try two OneHand keyboards: http://deskthority.net/workshop-f7/onehand-20-keyboard-t6617... It's a really neat design, but you'd have to source all the parts yourself.

That said, most of the convenience advantage of the Atreus over the Ergodox comes not from its smaller size but its one-piece design. You couldn't place two OneHands in your lap and type from there; it's a lot more difficult to stabilize if you're in bed, on the couch, etc.