I am still in the process of sorting this out.
Another question - not tandem specific:
In order to prevent or reduce the occurance of the chain rubbing on the inboard face of the big ring (when the chain is on the small ring) what would happen if one added some thin (say 0.5mm) shims to move the big ring further outboard or the small ring farther inboard? Will this mess something else up entirely? Is the offset from the small ring to the big ring a fixed number? Will this make shifting harder?