Joint Debtor and Co-Debtor sounds like the same thing or same debtor, but they have distinct meanings in bankruptcy. A person who owes money along with you on a given obligation is your joint debtor; whereas, a person who files bankruptcy with you is your co-debtor. For example, a parent who co-signs a student loan for you is your joint debtor. Your wife who files bankruptcy with you is your co-debtor.
You can be a co-debtor without being a joint debtor on anything.