Current state:
Both devices are working. (you need to add one wire between the silver holding clips of the usb port side (or the other ) to make the headless boot)

But it isnt ready either.
The biggest Problems are:
Fast power efficiency Connection between those devices.
- usb is fast but a real power problem. In hostmode one device charges the other. The connectors are weak and/or in a "bad" spot.
- serial is really slow and the connectors are "hidden" below the battery
- bluetooth is slow and consumes power even if nothing is transfered on both devices..
- wifi hotspot eats your battery too and is slow compard to usb and sdio
- sdio there are no wired powerefficient fast sdio connections cards yet. But are hopefully coming.
- i2c (https://talk.maemo.org/showthread.php?t=95745) you have to mod your working device and get afaik 400khz

The second Problem is software.
- If you run maemo you dont escape the "old software" problem but you have decent battery life and tested software.
- If you run debian n900 you have the "current software" problem
(there seems to be no good touchscreen and power optimised os for the n900/ n900 specs/ at all?) and you have a not so good battery life.

Finally I dont know how the close second device affects the radios of the first.