2 in 1 with one switch only

[Fernyfern]

Hello!

I am working on a pedal to use as a standalone amp/cab sim with just one 3pdt switch.  It's a ROG Thor into a multicab sim.  Problem is, I get a ground hum at higher gain.  I have used Tonepad's wiring template, so a star-ground on the input sleeve.  The DC input, the 3pdt switch, both boards, and the output jack all go to ground.  The  out of the ROG goes into the multicab sim in.  It all works except for that ground hum.  Should I connect both boards together with a ground?  Is there a wiring diagram somewhere for a 2 in 1 with only one 3pdt?

Any help would be appreciated. 

[Aleph Null]

Are you able to provide schematics for the two circuits, or a photo of your wiring? Those will probably make it easier to troubleshoot.