The advantage to having a first and second mortgage equalling 100% financing is that you would not have to pay PMI, which would be required on a first mortgage at 100%. The second mortgage is subordinate financing, meaning it is in the second lien position on the house, and therefore does not affect the first mortgage lender's ability to persue the subject property in the event of a default on the loan.
The thing to consider is that when you do this on a purchase, your first AND second mortgage lender will qualify you at the cumulative mortgage payment.