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.

The list of potential causes already mentioned in this thread seem more likely.  Given that the very first post practically advertises that multiple should work together*, perhaps a list of recommended power supply + recommended hub + recommended drivers + recommened cgminer build + getting started steps** should be included.

* which to me suggests they very much anticipated for users to buy multiples of to run together:

** Should they be connected in this order, or in this order or in a different order altogether?  Does it even matter?