I have a Focusrite 2i2, two inputs.
For playing, I usually go out 1 and 2 from QC into inputs 1 and 2 in the Foscurite and monitor it from there.
For recording : How can I record a stereo guitar + dry signal? Do I need to use the USB connection for that? In that case, how should I set up the routing?
To record stereo + a mono dry signal I think you’re best off to use the USB of your Quad Cortex. You’ll need to set up your preset so there is a dry output to 3 (mono) (or 3/4 if you want stereo) Try assigning output 1/2 to a stereo track in your recording software, and then output 3 (or 3/4 stereo) to a different track in your recording software.
Option 2 is one that is less likely to work: Try combining your 2i2 AND the QC as audio interfaces. Output 1/2 from the QC into the 2i2 to a stereo track, and the dry output from the QC USB to another track. I have doubts about this working, as most recording software forces you to choose ONE interface only.
You can set the imput as the quad cortex and the output as the scarlet if you want to use both , make two tracks , one is di 1/2 and tge other is wet 3/4 then assign what ever sound like to the di track or a plugin etc