Can I separate my ex's part of a loan, the divorce decree says she is liable for half?

A divorce decree is a court order. But that doesn't affect the bank. If And B have a joint loan and A gets it in the divorce, but fails to pay, B still owes it to the bank. B may go after A for failure to follow a court order, but B still owes and will still get a credit report entry.

Can you separate interest for taxes? That is an IRS issue, not a banking question.