The only ones who can answer the first question are your school officials. Different schools, different rules. Start with your department, and if you don't get an answer, move up to the Dean's office.
Outside committee members (not necessarily co-advisers) are fairly common, and in many cases, required. Their function ranges from fully involved committee members to someone that gets the dissertation a few week in advance and then appears at the defense. The latter can really turn into a rubber stamp, where the person doesn't feel like they can provide substantial input without taking the train off the rails, as its too late. I don't care for those situations.
If you plan to have an outside coadvisor, and its within your institutions rules, I advise frequent communications to your whole committee, with a carefully planned meeting schedule. So -- basically the same as with a fully internal committee.