Sam,
Sorry but only two cameras is not enough... I'd like to use up to four cameras at the same time and sadly ZWO doesn't allow for that.
Cam 1: Main Imaging on SCT
Cam 2: Guiding
Cam 3: All Sky
Cam4: Secondary imaging on piggyback refractor
Currently I own ZWO cameras for all of these roles, but since I can't have them work at the same time, I'm forced to use another manufacturer's camera for imaging and turn off my allsky camera while I'm imaging.
Pretty much every other vendor (Atik, QHY, Altair) allow for up to four cameras to work at the same time through ASCOM.
This is a serious disadvantage for ZWO and I'll expect it to be corrected soon.