OVF format is an easy way to migrate a VM to different hosts.
- Setup a common media between your two Ubuntu guests (e.g. shared
folder, USB mounted volume, network based directory, etc).
- Launch the Virtualbox GUI within the first Ubuntu guest (cmd is virtualbox)
- Select the VM you want to move (In the left hand pane, find it by name & click on it once)
- Choose "Export Appliance..." from the File Menu
- Follow the wizard (most defaults will be fine). Specify the file destination on your shared media that you setup in step 1. (e.g. /mount/dev02/sharedvol/nestedvm.ovf)
- Once it finishes, stop your Ubuntu VM and start your second Ubuntu VM (the destination for your nested VM)
- In the second Ubuntu VM, mount your shared common media from step 1
- Launch the Virtualbox GUI and choose "Import Appliance..." in the File menu
- Select the ovf file you created in step 5 (e.g. /mount/dev02/sharedvol/nestedvm.ovf) and follow the wizard to import the VM. If you plan on continuing to use the nested VM on the first Ubuntu server, you should choose to reset the MAC addresses.
If you are command line oriented, see the import and export options on the VBoxManage command in the Virtualbox manual... http://www.virtualbox.org/manual/ch08.html#vboxmanage-import
If you cannot launch the GUI, see this note... https://unix.stackexchange.com/questions/96997/open-virtualbox-gui