in order to install a single wide door in place of 2 singles, major reconstruction of the garage is necessary. when the central support between the doors is removed, a new support beam that spans the entire width of the new opening must be installed, normally including a steel I beam to get the necessary strength to span the 16 plus feet of the opening. the I beam will require a crane to lift from the delivery truck to the top of the opening. if the new opening is not 16 or 18 feet wide, the opening will have to be narrowed to 16 or 18 feet, or else a custom built door will have to be purchased. I would guess that the cost of such a conversion would easily exceed $10,000, and could exceed $20,000 depending on the construction of the existing garage. this is NOT a DIY job unless you have access to construction equipment.