Here's my setup of my Virtualbox. I have a Windows XP as my host and Ubuntu as my guest OS. I'm wondering if it is possible for 2 Ubuntu guests in Virtualbox to talk to each other. For example, ssh to each other?
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As long as you have networking set up correctly, they should be able to talk to each other just fine.
If you want them to communicate with the whole network, set each guest to bridged, If you have DHCP, they should both get their ips and work fine, you can then do whatever it is you want.
Adding in bridged mode is the quickest and easiest way, however you can also use one of the other networking modes and make them appear on a private network so they, and only they can communicate between each other.
In addition to my post - I just found this blog entry of someone attempting to do exactly what you are trying (vista instead of xp)- http://opensourceexperiments.wordpress.com/2008/04/18/virtualbox-case-study-making-host-only-networking-work-between-two-ubuntu-guest-os-virtual-machine-on-windows-vista-host/