First of all you mention french doorS, and to have a pair of doors that small and custom made would not be such a good idea as the confusion generated in a fire for such a small opening requiring both doors to be open wouldn't be compliant with the more current building codes and fire codes sprinkled throughout the USA and Canada.
If you're intent on installing a PAIR of such doors, you would have to have them custom made.
If you install a single French door-style swing door, you most likely could modify a stock size to fit your application, especially if it were solid wood AND had the more modern (beefier/thicker) kick-plate and frame style.
Standard Doors are now rather uniform in their size for door pannels.
They begin at 18" wide (or 1-foot, six in the lingo), the next size being a 2-foot (24"), the next sizes are increases in 2-inch measurements. In the trade they are called/sized by the number of feet followed by the number of inches, i.e. a 2-4 door is 28" (2-foot plus four-inches). Now it is common for the door itself to be just a bit LESS than that dimmension to accomidate a framed opening that is exactly that dimmension (making room for the hinge plates, etc.) but not an inch short.
Perhaps when you re-measure your door you will find that it is a 2-6 door (30"), or 2-8 door, (32"), or that you could plane down a 32" door to fit your 31" opening?.
When upgrading doors especially for fire-exits they are required to be a minimum width for escape and are to require NO SPECIAL knowledge for their operation. A pair of doors requires that one be able to exit by only operating ONE of them not both for egress. Your local building department, or Fire Inspector can advise you regards to which codes are in effect for your location. Even if your local building department doesn't regulate your replacement; you should still check with with your fire department regards to Fire codes, as your home-owner's insurance may not pay a liability claim if your modification resulted in death/harm to someone trapped in your home during a fire.
NFPA and ICC publish codes on the subject as well as other code bodies. Check with your local offices to determine which code and which VERSION of that code applies in your area.
Good luck to you.