I’ve been using an ASI6200 and ASI2600 simultaneously for several months now. When I first got the cameras I would get a dropped light frame every now and again, and I still occasionally get a dropped frame when taking flats or other frames of short exposure duration.
To start troubleshooting the problem, I began by finding a reliable USB cable that worked 100% of the time with the camera connected directly to my NUC computer. Don’t rely on the ZWO supplied flat cable. I went through 3 cables before I found one that worked 100% of the time. (I’m grateful for Amazon’s next day prime shipping!). Once I found a cable that worked, I then tried it through my powered, high quality StarTech USB 3 hub. The ASI2600 worked perfectly with no dropped frames. The ASI6200 would still occasionally drop a light frame and a little more often when taking flats, but it was much better. So, now I run the ASI6200 cable directly to the NUC, and the ASI2600 through the hub and everything plays nice together, with a very rare dropped frame during taking flats.
There does appear to be some sort of issue with the ZWO drivers or SGP, or the combination of both, but many people are using these cameras without too much issue. There are many other factors than just the software: cables, hubs and the computer you are using could all be the problem also. It can be maddening to try and pin this down, and I wish I could offer more of a concrete solution, but this is what worked for me.