You can still attach the keyboard and mouse using USB passthrough. That way, you make sure that the keyboard and mouse are dedicated to the VM and won't leave the Guest OS to interfere with the host.
Just make sure that the VM runs on the primary monitor, in case the secondary doesn't work as it should.

For Linux, you can, as noted before me, use seperate X settings. You can also use the dual monitor setup to create a big desktop (like you can with Windows too, and is needed in case you have Windows as Host). Only downside is that the other user can interfere with the Guest .

https://forums.virtualbox.org/viewtopic.php?p=6049 1