A good rule of thumb for double door rough openings is door width x's 2 plus 2". For example, if you have an interior French door you are installing that has two 30" wide doors, the two doors take up 60" and add 2" more for the jambs and adjusting room. Height is the same as single door openings and for 80" tall doors 82 1/2" is the correct size.
That being said, every rule is meant to be broken. If the door you are installing happens to have an astragal like in the picture. Most often you will need that extra width to the rough opening. That dimension is around 1/2" to 3/4" larger than that rule of thumb mentioned. A better suggestion is to have the door or verify from the manufacturer the RO size needed
So why or when would a door need an astragal and what do they do? A double door that leads into a bedroom or bathroom that requires privacy would be built with the astragal. This gives the main operating panel of the door something to rest against and acts as a door stop by covering the gap between the doors. This style also has locking pins on the top and bottom of the secondary panel to hold it snuggly in place. One side of the door operates as a single door and the other can be opened to double the opening once the pins are released.
Closet doors typically operate totally independent of each other without the strip down the middle. This allows both doors to be pulled open at the same time, however, locking the doors cannot be achieved as with doors noted above. Often ball or roller catches are used to keep the doors in a closed position. Watch a complete installation of a set of double closet doors.