Write down these words: Short Pay. The truth is, many times the second can't do anything but watch your ship sink and then come after you via debt collection. They will hound you with calls week in and out, warning you that every call is an attempt to collect a debt. Eventually they may well take you to court. If the judge bangs his gavel in their favor, you will owe the full amount. Your only option is to declare bankruptcy at that point.
Now, the thing to do is negotiate some sort of "short payoff." This can be dramatic in your favor. You might have a $30,000 second mortgage. Pay just $5,000 and get it in writing that they will free you from any deficiency. If you can get this done, the lien is closed out. Your primary lender then will more likely work with you on a short sale, because there is no third party involved.