If you have two AntMiner U3's where they both work fine on their own, I cannot imagine how thermal management would have anything to do with them not working fine when both are plugged in - unless both were put into an enclosed box together significantly raising each other's temps. Given the thermal mass involved, even that scenario seems unlikely to throw errors practically immediately.
It's not a "thermal management" issue. There are dielectric effects of having excess paste covering CCA components that should not be covered. I didn't believe it either but the results are real.
What results do you speak of? Did you have more than one U3 that wouldn't run together, then you cleaned up the paste, then they ran together just fine? I have not seen any evidence of this in this thread. In fact, I haven't even seen evidence of more than one U3 running together on the same machine without issue.
You haven't read my posts then. I had 3 units at one time and none of them ran together. After cleaning up the paste performance improved but I still had problems. I've since sold 1 unit, but since replacing the USB cable I successfully run 2 U3's and 1 Rockminer R-Box together. Every 4 days or so I do have to cycle power on the U3's and unplug/replug the USBs, but that is acceptable to me.