In open space, two boxes in normal mode will provide more output up front than if one of them were in cardioid BUT if a reflection from a wall behind the subs was to cause a reflection that cancelled the primary output of the two subs, then placing one of the cabinets in cardioid mode to mitigate that reflection could result in higher SPL in front of the two boxes. In other words, you can use cardioid mode with just 2 boxes to good effect when it's called for.
Cardiod mode uses the output of one of the boxes (which is turned to face backwards) to deliberately "phase out" the sound going backwards behind the stack. If the sound going backwards hits a wall and then comes forwards again, that sound can "phase out" the forward-traveling sound. If you cancel the backwards-traveling sound before it hits the wall, it won't have the energy to do any cancelling out front.