Check monitor orientation
I haven't done this in a long time, so I could be wrong. I think the start menu will only extend if you have the monitors oriented (in the display properties) so that they are right next to each other. Then again, it may not be possible at all - the best you may be able to do is to create additional menus (like the address bar) and place them on the secondary display. You should also check the start menu properties & search on groups.google.com -- I'm sure that someone does this & that it's really easy.