If You are a PC user, remember to stretch the actual application window to cover both monitor if You e.g. want to have the rack on one monitor and the sequencer on the other. Grab the lower right corner of the actual application window and drag iacross to the other monitor.
If You use a dual monitor setup on a PC, and Reason seems to be hanging when You try to save Your song, this is because of a bug in 2.0 that has been fixed in the 2.01 release. In fact Reason does not hang at all, what happens is that the Save (or Save As) dialog opens up on a screen coordinate that You cannot see - it's outside Your actual screen areas. If You type a few letters and hit the Enter key, the song will be saved and You can continue to work. Download and install Reason 2.01 and this issue will be sorted.
On certain dual monitor setups, the contextual menus and the tool tips will be invisible for the same reasons as explained above. This bug is also fixed in the 2.01 release.