I think that there are different methodes to formulate dualtouch on a singletouch device. (My propositon is still touch&swipe).

The problem for the moment is only how to make believe the n900 that he is a multitouch device.
Then you can send emulated multitouch events to the n900.

Probably you need to find a multitouch driver for the n900.
That should be no problem. Because there exist usb-multitouch touchpads that can be used with arm-linux.