What LZ has asked is called "bi-weekly mortgage."  It reduces number of years you pay off the mortgage.  There are few mortgage lenders who will accept half payments so third-party services are usually the ones who set up biweekly mortgage plans.  However, there typically has fees associated with such service.

"The faster a borrower pays back a mortgage, the faster the borrower builds home equity. A biweekly home mortgage is a mortgage loan plan that helps the borrower achieve this goal. The biweekly mortgage allows the borrower to divide the monthly mortgage payment in half and pay them every other week. Because there are 52 weeks in a year, the borrower ends up making 26 payments, which add up to 13 monthly installments instead of the usual 12. Because of the extra installment per year, a 30-year biweekly mortgage is typically paid off in about 23 years and a 15-year biweekly mortgage is paid off in about 12 years."