Looking to answer a question about flight controls, I stumbled across this site. It has, in Russian, the Po-2 operation manual.
(And to answer my question, yes, both cockpits seem to have full flight controls so the navigator can perhaps land the plane if the pilot is taken out.)
And they did- there’s an illustration in the book based on a real account of a navigator flying from the back while holding the dead pilot to keep them off the stick.