How to improve the insulation seal on French Doors to stop drafts?
You found the weak link in weatherizing French doors. I've found that it's as much art as science. Where the two doors meet the jamb is often a problem.
In the pic it appaears that the weatherstrip above the right door is offset from the vertical strip on the left. Did you try simply repositioning the weatherstrip where you have the gap. Sometimes the weatherstripping has peel and stick adhesive on the back side. See if you can reinstall it to conform to the door and fill the gap.
If that doesn't work your local home store should have any number of pliable insulation strips that you can attach to the exizting strip in that corner. At worst, replace the entire weatherstrip - they're pliable and can be made to fit.