I have:
1 Compac
1 2pac
4 2pac Seconds
1 Newpac

All currently running at 150 mhz.

They are on stands with active cooling and the normal USB 5V is bypassed to use a separate 60A 5V power supply.  (I have no power issues from crappy hubs - they get all the juice they need)

Two of my 2pac Seconds are flaky (the good one just zombies often, and the bad one has a solid white light but still hashes about 75% as good as normal and also zombies often), so i would like to run them at a lower speed - they tend to run longer before crapping out at lower mhz.  aside from just running those on a separate pi, is there a good way to target it's serial number or something?

i know there is the --USB command, but as far as i can tell that just picks up whichever one happens to be listed first?