Sorry, I think you misunderstood the manual.
Connecting a cable to the input/output jacks of the H90 will determine wether it will process in mono/stereo.
If you use Dual Mode, you can have two paths; Inputs/Outputs 1-2 for Path 1, Inputs/Outputs 3-4 for Path 2. You can use Path 1 for your guitar amps, and path 2 for your audio interface. You will need to enable Dual Mode from the system settings: https://cdn.eventideaudio.com/manuals/h90/1.9.7/content/system-menu/global.html
