In Views, how can I show only the results for content that has been tagged with two taxonomy terms?
I'm not sure why are you using Relationships for this. Simply, from "Contextual Filters" section, create a "Content: Has taxonomy term ID". In that filter's settings -> "More" settings -> Check "Allow multiple values" which: 


  If selected, users can enter multiple values in the form of 1+2+3 (for OR) or 1,2,3 (for AND).


i.e. go to view_path/29,34 will filter results by both terms.

Hope this helps.