Hi There Bbomom, thanks for the question,
In a short sale the primary mortgage is wiped out. This means that the lending institution will write off the loan on their books. They will then send you a 1099-C(cancellation of debt). This must be reported on your tax return.
The subordinate loans; 2nd, 3rd, 4th.... are not directly part of the foreclosure of your primary loan. Your Realtor or your attorney will have to negotiate directly with the entities that hold these subordinate loans.
So in answer to your question, Yes you are responsible for the balance on the second. Tell the realtor or the attorney that is helping you with the sale to contact the holder of the second. It is important that the second is handled correctly during the short sale process.