You can cut them yourself, but how you do it depends on the type and style of the door. For instance if it’s a hollow core door (solid structure around only the perimeter of door) you will need to add in a solid structure on either side of your cut. One method for doing this is ripping down a solid board, lightly gluing it, carefully inserting it into the cavity of each section of your now two piece door, flushing it up with the cut edge, and pin nailing it in place. A helpful tip in doing this is to run a screw into the filler piece to hold onto and help position it with out it falling down into the cavity of the door. Once you have it nailed remove screw and fill hole. You will also need one new hinges to each section of the slab and door jamb. The best way to do this is with a hinge jig and router. You will be left with the old shim locations on slap and jamb that will need to be filled and sanded.
On a raised panel door the position of your center rail will determine where to cut it or split the slap. It will be a solid rail piece so you won’t need to do the fill in piece like in the above example just some sanding and finishing. You will however still need to add and relocate hinges following the same steps.
This can be a labor intensive project and it does require knowledge of jigs and routers, so if you’re not comfortable or familiar with these tools or techniques you may want to consider just replacing the unit with a pre-hung (already cut and mounted in the frame) split door. It may require going to more of a specialty building supplier like PMD (Pacific Mutual Door) vs. the big box stores. And although it might cost more, it would certainly take less time and sweat equity.
– Jeff Wedgewood, Siding Expert at Bordner