No it uses only one port... I'm using a powered hub but it is possible to have the n900 poer the device (i tried it).

Compiling... i needed to patch an older version of udlfb... anyway it was not working. For the newer Version i needed to change the Makefile afterwards it was up and running but showing nothing on X (wrong EDID). EDID in fact is a bad thing...

I'm now on the displaylink-mod driver where i needed to change the Makefile as well but it compiles and is loadable...

I tried nearly all displaylink drivers now... and all version combinations.
When the module loads, i get the green screen (for displaylink this means it is working). However Xorg from within maemo starts up but the screen remains green... i get an ioctl eror from omapfb module...

anyway as the kernel module runs now i will install the whole X thing inside of debian to give it a go.

And perhaps i can help you beause i bought a 'docking-station' on ebay - i'm unsure but i think it uses the sisusb....

cheers,
chris