You should do a short sale to save your credit

va_landlord is right.

You don't need to put any more money into the house. If you agent does not know how to do a short sale, hire another agent.

Stop mortgage payment, continue to collect rent, and tell your bank that you want to do a short sale. Hire an agent who knows how to do a short sale, and put both houses onto the market.

The worst thing you can have is that your bank does not approve your short sale, and go ahead to foreclosure. You are prepared for that anyway.