I ve installed many pre hung interior french doors with great success.
How much of a gap between french doors.
Nail it to one of the doors using small wood nails so that it overlaps the other door by about one half inch covering the gap.
The gap around all the doors and in between the two doors needs to be consistent and as equal as possible.
Turn the upper hinge adjustment screw to even out the gap.
When calculating the width remember there are three gaps between the doors instead of two plus a jamb on each side of the opening.
So if the doors are 24 inches wide leave a 1 4 inch gap between doors and between each door and the jamb.
Double interior french doors are hard to install because you have additional tolerances you must satisfy to make the install look professional.
Do this on both doors.
Unlike interior doors gaps between a front door and a frame must go with the standards not so much due to the possible expansion of a leaf but because of the danger of penetration of cold air and extraneous sounds from outside.
Because one or both of the french doors are moving many traditional weatherstripping solutions just don t apply.
Close the gap between the two doors with insulated molding.
Cover the nail heads with wood putty and sand them smooth.
It s not uncommon for a gap or crack to exist where the two doors come together.
Place nails about every 5 inches.