How can I get the second party to pay on a loan that was opened as joint account?

Simple answer (and not to sound flippant): sue them.

It's impossible to be any more specific, or even guess at the potential outcomes without more information, including analysis of the agreements, etc.

You will probably have to sue the other party to make them comply with the terms of the agreement. Sometimes the court will make you have to sue every month (if the terms of the loan are monthly payments) that the other party does not pay.  Technically, they are not defaulting on their agreement until after the due date for payment.  I would go and see an attorney and bring the loan agreement.  It may have terms and conditions that govern you and the other party as well.