You can record both a microphone input and a guitar input at the same time even though the USB3 shares an input for the mic and guitar input. To do this, you will need an external amplifier, a mic preamp for microphones or a DI box or other Hi-Z interface for the guitar, and then take the line level signal out of that into one of the two 1/4" line inputs on the back of the USB3.
If you want to use our proprietary E.F.R.™ Guitar Input circuitry, then you‚ll need an external mic preamp and take the line level output of the mic preamp into one of the USB3 line inputs.