In almost every case,were the home to be foreclosed by the lender of the 1st mortgage, the holder of the second mortgage would get nothing. Typically a 2nd mortgage holder will be offered 10% of their mortgage balance, or $10,000 flat, in return for their agreement to a short sale. It is unusual, but not unheard of, for a second to refuse those offers. That's usually the case when the second comes from different lender than the first. If that's not the case, I'm mystified.
That said, everything in a short sale is negotiable, and there's nothing to prevent the first mortgage holder from accepting your offer but demanding that you pay the second. It's reallly no different from any other offer-counter offer scenario. You can accept the seller's offer, reject it and go away, or make a counter offer.
I have made a lot of assumptions here without knowing the facts of your situation and I certainly agree with all who have said you should consult a real estate attorney. If you don't know of any let me know and I will give you a couple of names.