If another SUB goes on your Z-Gemma Box, this can possibly cause problems, seen it plenty times. It also Depends also if it is via the same method getting your channels populated via your bouquets for both the SUBS..
SO, best way you can do this to have 2 separate subs running along side each other in a safe and reliable method is to Install via a Plugin for the 2nd SUB and leave the other sub via the original Method. How ever they supply you on the original SUB?
Script, M3u, SULS etc?
SO, get the extreme TV Plugin or XC Plugin and install the other sub via this method, you will then be able to have 2x SUBS on the same box with 2x different suppliers
Not to sure why you would want to do this, but hey ho.. it can be done.
Hope this Helps