I never heard of anyone mining a U2 directly with a Pi lol
A Pi is a memory control card, you use one for miners that don't have built in miner software; it's not meant to power a Bitcoin miner.
The best way to set those up is to get a regular USB hub and a fan, MinerSource.net has awesome ones

You'd use a Pi for miners like RockMiners, a USB miner can be set up with a program like EZMiner (or whatever the name is).
Miners that have their own power supply, require very little power from the Pi...
So try it with a good quality USB hub and see what happens!
Hope this helps and happy hashing
