Its is possible with virtual box, but its not exactly well intergrated since it was never a feature that it was meant to do, so you have to tinker with it to get it to work. I have done it before so it is possible and it works nice when you have it all setup.
What you have to is connect 2x keyboard and 2x mouse to the main computer. At this point, both items will control the pointer on the main computer. What we need is for the main computer to ignore input from the second set.
So what you have to go into device manager and find the 2 new input devices in the and disable them.
Note: You are disable the devices themselves but not the USB controller they run off of.
In virtual box, you have to capture (pass-through) from the USB controller. When the USB controller is passed through, virtual box will recognize the 2nd set of inputs while they are disabled in the main computer.
Virtualbox will always autocapture the usb hub, while the components will always be disabled on the main computer.
This setup works will if you never disconnect any of the components. If you have to move the interfaces around, you have to redo the process. Also the 2nd computer will not have GPU accelerated features so forget 1080p video, animations, etc unless you want to rape the cpu. I hope they figure out how to pass a gpu hardware level.
Yes you can use an additional free output from any open port, and run virtual box full screen, but GPU features will always be CPU rendered.