Views: filter content by two taxonomy terms
Create three contextual filters: 1) global null (it's at the bottom), this will ignore the first argument 2) normal contextual filter for the first vocabulary, 3) normal contextual filter for the second vocabulary.

By normal I mean that you first select Term ID then Specify validation criteria, Taxonomy term, pick your vocab, Term name converted to term ID. It's a good idea to give these filters an Administrative title found under the More section, so that you can keep track which is which, as they need to be in the right order.

To reduce duplicates, in the view details "Advanced" > "OTHER" > "Query settings", check the box "Distinct" (thanks user2171413).