I am using cgminer with a NewPac miner in a powered USB hub. Overclocked I can get about 100gh/s.
I got a 2nd USB miner, configured the proper driver, and reloaded cgminer. cgminer detects both devices, but both sit around 50gh/s.
I've never used USB miner, but I've read somewhere not long ago about similar problem and iirc one possible problem could be that the USB hub doesn't provide enough power for both miners.
So this would be first thing to check. Do you use Gekkoscience hub?
Even more, I suggest you read
the Gekkoscience Newpac related thread and also ask
there. It should increase (greatly) your chances get answers from people actually using exactly that miners.