How to get my exterior door to close when the interior door is also shut? Air pressure problem.
Before concentrating on an air pressure solution, I would eliminate two other possibilities:


The handles on the inner and outer doors are interfering very slightly with each other.  Check the height of each handle above the sill, and the spacing between the two doors when closed;
The act of closing the inner door distorts the door frame slightly, shifting the outer door from the "fits well" category to the "fits with great difficulty" category...