Search for a Secondary Mentor.

I think the roll of a secondary mentor varies depending on the project, the student and the primary mentor. Honestly, I think the secondary mentor should be someone the primary mentor finds or at very least approves of before signing them on, simply because I think it's immensely important that the two mentors interact well. Otherwise, the student could find themselves stuck between two conflicting mentors which, I would imagine, would not be pleasant.

For my project, if I would have a secondary mentor it would be someone to help take the weight of teaching me everything about HCI off Prof. Mitchell's shoulders. 

As I stated above, I believe the primary mentor should be the one to locate the secondary to ensure compatibility between mentors. However, you can locate a secondary pretty much anywhere you like. They can be a faculty member from a different school, they could be an expert already working in the field you're working towards, the sky's the limit on where you could locate one.

Also stated above, I feel like whoever the secondary is, they need to be compatible with the primary, otherwise the student could find themselves in an awkward tug-of-war game between the two mentors.