These two very similar sounding terms have very different meanings.
The term joint debtor describes the second person listed on a bankruptcy petition when a married couple files bankruptcy. The first person listed on a bankruptcy petition is the debtor. There is no rule in the Bankruptcy Code that states either the husband or wife must be the debtor or joint debtor.
In bankruptcy, a co-debtor is someone who doesn’t file bankruptcy, but is liable for debts that are included in a bankruptcy because they share liability for the debt with a bankruptcy filer. For example, if you are married and have incurred debt with your spouse you are both jointly liable for that debt. If your spouse files bankruptcy, and you don’t, then you are a co-debtor in a bankruptcy case.