The operating system can use either processor for whatever it wants to. You can buy dualie boards from many motherboard companies, Tyan and Supermicro being a couple that focus on dualies in particular (or quad, mmm!

). Generally the boards use a particular type of memory modules and power supplies, and are bigger than many popular cases will accomodate. They may have multiple high-speed PCI-X buses and fancy onboard stuff like Ultra320 SCSI and one or more gigabit network controllers.
For a home user, dualies are good if you have multiple hard-hitting tasks that you want to run simultaneously, such as a video-conversion process running in the background while you're playing Doom3 maybe. Of course, a guy could just build two completely separate computers too...

but whatever.
If you want to scope out some dualie boards,
here ya go 