Can you tell us what you have done so far?
Are you trying to display one video output while capturing another? Or
are you trying to display two different video outputs while capturing
another? It's not clear from your description.
Would displaying one video output while capturing from your image
sensor work for you? If so, the xM should be able to do that without
any extra special hardware.
If you want to display two video outputs, then you may need to have
some extra hardware to enable a 2nd display (I'm not that familiar with
the xM video output ability). A SPI connected 2nd display may work.
Saving to hard disk shouldn't be a problem, either using the SD card or
a USB attached disk should work fine.