USB can not handle two audio interfaces at the same time. USB is not a "network", but a peer-to-peer conncetion. You will have to use either your Axe preamp or the StudioLive mixer as interface. It's not possible to link USB streams, by the way.
It could make sense to use the XLR outputs of the preamp, because TRS cables are automatically changed to "line inputs", which takes the mixer's preamps (Gain) out of the signal chain. And why not add Output 2 as additional stereo source?
I don't know if the AES input might be of interest. The StudioLive mixers can send a stereo signal via AES (output only), which can be Main L/R or any (linked) flex or matrix mix. Usually this would be used for feeding recorded tracks or playbacks.