Maybe I am crazy but the longer a cartridge is the LESS the angle of the hinge to breechface(top) should be to open the action the distance required to remove the cartridges. A short cartridge with a short hinge to breechface(top) length will require a larger angle to open the action to access the cartridges.

Of course you can blow all the previous verbage away by making your hinge to breechface length anything you want for short cartridges!!

Given relatively long cartridges the reason the O/U must be opened further vs SXS with similar hinge to breechface lengths is that you have the diameter of TWO cartridges plus the surrounding metal to move vs the ONE cartridge width + metal of the SXS.No doubt the SXS can be reloaded with less motion of the barrel to expose both cartridges.

I like the concept of the scoped O/U for longer range medium caliber cartridges and the stacked barrels with the top barrel closer/under the scope for longer shots.For medium caliber Doubles it seems a natural, for large diameter/calibers a liability.