Dual sims do work in the 900,

http://www.youtube.com/watch?v=rh57kQc9DRc

and talking to the gentleman who created the vid, there is a builtin timer you can use to switch back and forth btween cards. using a service like Google Voice; would eliminate the need to switch back as you could forward calls to one or the other sim card.
Since there is only 1 GSM radio in the device having a virtual radio would be the way to go.

http://talk.maemo.org/showthread.php...al+sim+adapter

interesting thread on Dual sims.