Not typically. A removable mullion sits behind the doors and are used with externally mounted exit devices and strikes where the exit device latches into the external strike mounted on the mullion whereas a fixed mullion is between the doors where the cylindrical, Tubular or deadbolt lock would latch into the strike mounted in the mullion.
NOTE: There are some hollow metal door manufacturers that may make a removable mullion (for their door/frame) that are designed to fit between the meeting edges of the door. For this type of opening with removable mullion, you should contact the door manufacturer.
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