How can I get VirtualBox to play nicely with dual monitors?

According to this forum post and looking through the VirtualBox release notes this is only available to Windows Guests. I know the latest 3 release introduced huge improvements on the 3D rendering side and with OpenGL. I would assume this would be available in the near future.

And according to the 4.0 release notes, the feature is available!

Guest Additions: support for multiple virtual screens in Linux and Solaris guests using X.Org server 1.3 and later

My solution involves running Ubuntu in headless mode and connecting through the Remote Desktop Connection tool. The display is full screen (no toolbars or menus) and spans both of my monitors. Here's what I did:

  1. Double video memory to 16 MB in VirtualBox display settings for the machine. Also enable "Remote Display" server. For some reason I couldn't use the default port of 3389.
  2. Run the machine in headless mode like this:

    VBoxHeadless.exe --startvm "Ubuntu 10.4 64-bit"

  3. Connect to the machine using the Remote Desktop Connection tool that comes with Windows.

    mstsc /span /v:192.168.1.2:3390


The fullscreen display in both monitors can be easily done.
Luke Dubber managed it and it is explained here:
Steps:
1 - Install VirtualBox (its free, Windows, Linux and Mac)
2 - Once you have created your virtual machine (VM) edit your settings.
3 - Go to Display section and increase the Monitor Count.
4 - Make sure to increase your Video Memory. If you don’t it won’t work (I should know, took me a bit to figure out that was my issue). I would go with the maximum 128MB if possible.
5 - Install your OS (you can do this before step 3).
6 - Make sure to install the VirtualBox guest additions.
7 - Then reboot and once you are back in you should be able to use more than one.
To read from original link Click here:
Or go to the home page of Luke Dubber

The seamless mode is something I'm battling with also and haven't seen an answer to yet.