VirtualBox 3.2.1 supports multiple guest monitors. The documentation was not clear on how to enable this.

Basic Setup:

Power off your virtual machine if it's on.
From the main VirtualBox window, select your VM and choose Settings.
Choose display.
Below "Video Memory" is Monitor Count. Slide it to 2, and adjust your video memory if VirtualBox complains.
Start your guest and perform the standard "Extend the desktop onto this display" method based on the Guest OS. (Guest Additions need to be installed.)
A second "Oracle VM VirtualBox" window will appear with the second display. You can resize it however you want.
Seamless/Fullscreen

Enter Seamless or Fullscreen. I'll assume your HostKey is the default RightCtrl.
If the screens are on the wrong displays, hit RightCtrl+Home.
From the View Menu, choose Virtual Display 1 and set it to the Host display you want. The other displays will shuffle around to accommodate this. If you have more than 2 virtual displays, repeat with Virtual Display 2 and so on.
Headless

Set the number of monitors with VBoxManage modifyvm "vm name" --monitorcount X
Enable multiple vrdp connections with VBoxManage modifyvm "VM name" --vrdpmulticon on
Use VBoxHeadless to launch as normal.
connect to monitor 1 with rdesktop -d \@1 ip-address-of-host and monitor 2 with rdesktop -d \@2 ip-address-of-host. This is explained in lomaxx's answer. (You might be able to use @ instead of \@, depending on your shell)