Part of the problem with friction shifting on a tandem is that you're far from the derailleurs and insulated by the stoker's pedals (cranks). That virtually eliminates the ability to shift and trim by sound and feel which friction shifting depends on. I gather you're using index for the RD. Consider giving FD control to the stoker, at lest as a short term experiment.

Otherwise you might use a separate clamp behind the downtube lever to improvise a bump stop for downshifting to the middle, and arrange it so some extra force moves the lever past for the granny.