How do you frame an existing, finished opening for double swinging closet doors to be installed?
You usually start with the current opening (if finished) and pick a standard door size, add in for gaps (3 gaps: Left hinge, right hinge and center).
Typical gap sizes would be 1/8 on either side and 3/16 in the middle
If the opening is currently drywall, the corner beads would be removed and a wooden jamb is installed left, top and bottom. Casing molding is installed over the jamb to cover the gap between the jamb and the original opening.
Roller catches at the top center are nice, because there is not a usual single door strike plate
You have a choice of door handles (which will need wider stiles) or cabinet pulls,