You need to use output 1 to the instrument input of the Soldano, not output 4.

in dual mode the path A is input 1/2 to output 1/2, and path B is input 3/4 to output 3/4.

In this setup, path A will be preamp in mono and path B will post in stereo and you can route one or both presets to either path in series or parallel.