Kind of a wonky suggestion, but are the ML1 and ML2 on different hubs? If you power off one HUB, then the others should be recognized sequentially? Then after the first set is running, perhaps you can turn on the other hub and have those USB devices recognized in the next set of sequences.

Thanks for this warning. I would have thought the MLD would recognize by order placed in the USB HUB, but it sounds like it is more of a first come first served type of situation? Not sure how far bfgminer forked from cgminer 4.10, but cgminer 4.10 lets your blacklist USB devices for a particular instance... so conceivably you could run two instances with specific hardware tied to each instance. The Manage Devices on bfgminer is very different from cgminer 4.10 though.