verbally accepted offer; got a second interview call I was made a verbal offer for a job at a college, which I accepted over the phone. I haven't got the written offer and I have not signed anything yet. I was informed the written offer will be sent to me in a few days.

That evening, I received an interview e-mail from the same college but a different campus (which I prefer).

What should I do? Wait till I get a formal offer letter from the first campus and then let them know I need more time?

What is the wisest and the ethical course of action?
You're not under any contractual obligation (and don't have any legal recourse if the college were to withdraw the offer!) until you've gotten a written offer (which will normally have a deadline by which you must respond) and returned it with your signature.  It's in your interest to continue pursuing other opportunities until the first institution has come through with a complete written offer and you've accepted it.  

I would contact the second campus, explain that you would prefer to work there if you got an offer, but that you've already got an offer in hand (and what the deadline on that offer is, if you've been given one.)  You need to know how rapidly the second campus will be able to conduct their interview and make an offer.  If they couldn't possibly make an offer to you before the deadline on the first offer, then I would decline to interview and just take the first position.  If it is possible for them to move quickly, then I'd suggest that you go ahead and interview.  

There is a risk here- if the first institution finds out that you're interviewing elsewhere, they might decide to withdraw their offer.  The only way to avoid this risk entirely is to turn down the second interview.