How to add a second Contextual Filter to the calendar view? (this view is automatically created by the calendar module)?
What are you trying to achieve exactly? You want to restrict the calender to show only nodes tagged with a certain term? 

It looks like you're trying to do this by setting up a relationship with the terms and then passing in the term name. Is there a reason why you're not using the Content: Has taxonomy term ID contextual filter? This cuts out the need for the relationship, but requires you to pass a term ID rather than a term name.

If there is a reason why you're not doing this, then ignore me :) Just thought I'd reply in case you didn't know about it.