Way back in hardware school we were taught that rim panic hardware with a removable mullion is the most secure and maintenance-free application for a pair of doors. Most facilities prefer mullions instead of vertical rod panic hardware, which can be difficult to install and to keep properly adjusted. On the other hand, most architects request concealed vertical rod panic hardware, to avoid seeing the mullion or rods. I think the benefits of the mullion far outweigh any aesthetic issues.
I have had many architects ask for clarification of where the removable mullion mounts, often questioning whether the mullion has to sit between the doors. The removable mullion actually sits behind the doors, where the rim panic hardware latches into it.
In the “Olden Days,” removable mullions were installed with screws, and when the mullion was removed to allow the passage of large equipment through the opening, it would often end up reinstalled with a screw or two less each time. The option of having a key-removable mullion makes it easier to remove and reinstall the mullion without jeopardizing security.
I received a link to this video the other day, and I thought it might be helpful to illustrate the location and installation of a key-removable mullion. Don’t forget that when you use a mullion you need to measure the clear width of the opening from the face of the door open to 90 degrees, to the edge of the mullion, which reduces the clear width by about 1″. Each leaf will need to provide 32″ of clear width if that leaf is required for egress or an accessible entrance.