my girlfriend and I frequently double up when we are traveling, and I can say that the creek hp amp will drive two pairs of grados (rs1 and sr325) well. I do believe there is a slight loss relative to listeing with one pair, but nothing like the loss when we split the output of our grado ra-1 amp. I suspect the loss would be worse if you are using more demanding loads than the grados. Still I think the creek amp is a good one for driving one or two pair.
I don't have two demanding headphones to try both slots at the same time with, but as wrtickle said, it will drive two less demanding headphones quite well. Oh, and mine is the 21SE, which I think might be more powerful than the standard 21 anyway.