They actually come pretty close. The 16 ga barrels will lock on to the 20 ga frame, and the 20 ga barrels go about half way closed on the 16 receiver.
Close inspection of the barrels indicates that the groove for the hinge pin on the 20 ga barrels is not as deep as on the 16ga barrels. Were it cut deeper, I think they would fit. Whether they would be on face is an entirely different matter.
