I've done this successfully but with completely different software.
You need a program to synchronize exposures. I had a program written for me that could synchronize theSkyX for one camera and CCDSoft for the second. The program would delay the start of the exposure until the dither was finished. If the exposures need to be different lengths a delay can be added to the shorter sub. The key is that both subs need to start at the same time.
Years ago I wrote an article for Astronomy Technology Today if anyone is interested:
https://drive.google.com/file/d/0BwR...48-cA4vRZzNKsw
Peter