Successful link of 1st Google Wifi Disc, 2nd one Won’t Connect

After reading the forums, I was successfully able to link the first and primary Google wifi disc to the art router through IP pass through. When I go to add a second device, the app only prompts me to “set up Google wifi”. It finds the second device fine, but then tries to have me join it as a new network (which clearly won’t work because it is not the one in pass through mode). What do I do? How do I get the second device added without having it try to set it up as a new network?