You can not use two video cards at the sme time so dont try it. You could if you had a ZII card, but its hard to get working correctly.
When you get the Mediator and the MMCD just email support@elbox.com with the serials off the board and the CD to register. Dont tell them you got them used, they dont care as long as you have the valid numbers. I have heard people not getting support when the tell elbox the items were bought used though. So just dont say to be safe.