Abstract: A mulling system includes a first joining plate with a first plate member, a second plate member, and a first thermal breaker that extend longitudinally along a mulling axis with the first thermal breaker attaching the first and second plate members. The mulling system includes a second joining plate that includes a third plate member, a fourth plate member, and a second thermal breaker that extend longitudinally along the mulling axis with the second thermal breaker attaching the third and fourth plate members. The second joining plate is configured to attach to the first joining plate to join together first and second fenestration units with the mulling axis extending between the first and second joining plates. The first joining plate substantially matches the second joining plate to define a corresponding rotated and engaged arrangement of the first joining plate and the second joining plate across the mulling axis.