Stable doors on houses are normally rebated together in the middle, these are normally framed style doors opening inwards. If they are on stables or sheds they are often just brace and batten construction, there is just a bevel where they meet, they normally open outwards so the bevel keeps the water out. They can be made as 1 big door and cut in half, everything is guaranteed to line up then, you just leave a few mill gap between the two middle battens when you make it. A track saw is the perfect tool for cutting them in half.
Doug