If there was a feature request for Virtual PC or VMWare, it would be dual-monitor support. Why hasn't this been done?
It has, the VMWare workstation allows you to expand the veiw across two monitors.
The little tab at the top of the screen has a button to expand across both monitors.
See page 159 http://www.vmware.com/pdf/ws65_manual.pdf
Virtualbox appears to have this feature: http://forums.virtualbox.org/viewtopic.php?t=3166