Vidiot only has one video input decoder. You would need another LZX device (Visual Cortex, Cadet III, TBC2) to get a second input.
BUT, if you just want a video mixed into the feedback loop, it is probably easier (and cheaper) to introduce a (non-LZX) video mixer in the chain. If you get one with luma keying (Edirol V4 is sort of the standard, but there are definitely cheaper options), you can have the video output from the computer on one channel, key out the lights or darks, and have the screen feedback from the Vidiot playing on the other channel in the keyed out parts of the image.
There are also ways to make this happen sans hardware mixer. But you will then need some hardware (like an Intensity Shuttle; though again, cheaper options exist) to get the Vidiot’s output into your computer, and software (you might check out the Signal Culture apps, for instance) that can mix that with the video and output to an external monitor.