The Cylinders have a very small divider in what is basically a sinlge opening. It is non structural to the engine block and I have seen some of them removed to create a single port with no ill effects. Even with the 12 openings per side, the side consists of 6 stacks with two openings per side that butt up to the small divider wall. The guys who removed the divider wall also removed the divider between the ports and used single port per cylinder stacks. There was no difference in performance, but the external appearance changed slightly, as you might expect.
Technically you can consider it two ports per cylinder, but they go into the same cylinder. If you look into both, you are looking at the same place. That's why I called it one port. Also, I can stick my index finger into one port and put it into the other port from inside the divider wall, so it doesn't do much except increase back pressure. The exhausts with one opening per stack have a 2-into-1 collector ... except for the custom stacks I saw once where the owner removed the center divider and improved the exhaust flow. Externally it looked very similar to a stock unit, but the footprint was difference where the stack surface seals to the cylinder bank.
So, it's a matter of perspective and I suppose you CAN consider it a double port ... I just don't look at it that way since I installed one stack for one cylinder regardless of how many openings it had.
I've also seen Merlins with two cyinders feeding one exhaust opening and they look like big six cylinder unit because there are only three openings per side. I believe this was prevalent on early Hurricanes ... but the stacks were being fed from 12 cylinders. Then there are the one-opening per cylinder stacks.
A Hurricane, Spitfire, and P-51D or later Mustang all have Merlins, but don't especially sound similar to one another due to stack configuration. a Griffon sometimes sounds like an engine failure about to happen due to the stacks, especially the Fairey Firefly with night fighter exhaust ramps on it. Over here in Southern California, we have one owned by Captin Eddie and it sounds like a real hot rod compared with the Merlins we usually hear around Chino. Hopefully it is approaching flight again after a gear-up landing caused by gear malfunction ... they tried for an hour but couldn't get the gear extended, so they foamed a runway and slid down the foam. The real issue might be finding a new prop!