hi,
if you have seperate outs on your USB mixer, yes you can.
For example the Presonus FIREBOX has a 6X8 interface, i could route a signal or even the main mix to two unused outputs and send that to another mixer. depends on what you want to do.
i think it'll be good, like what soft said, if you'll give more details on what you wanna do with that seperate signal. like that, we can help you better with solutions on that.