Any time that you agree to be liable on a debt with another, you have probably agreed (absent some limitation in your loan document) to repay the entire. It's generally called "co-signing" or "co-borrowing" debt. So, from the creditor's perspective, you both are equally responsible for repayment of the entire debt. But, as between you and your co-obligor (your ex-husband), you may be entitled to reimbursement for any payments that you make on the debt above and beyond your fair share. Here,...