Strange as it sounds, it's because of their engineering plant. They have electric propulsion with the motors located close to the propellers and no propeller shafts in the conventional sense. Power is provided by four diesel generators and two gas turbines, one of which is located in the sponson under each island; you can see the funnels in the islands. Gas turbines need large volumes of air and generate large volumes of exhaust, which would take up a lot of internal space if they were located in the normal engineering spaces. This also means the turbines are isolated from underwater damage, while the diesels deep inside the hull under each island are protected from missiles, bombs, etc. And of course the engineering plants are about 300' feet apart. This all limits the amount of the ship's generating capacity that can be knocked out by a single hit.
The arrangement has some incidental benefits like providing more locations for radars, electronic warfare systems, and communications antennas to avoid mutual interference. We might note that our Nimitz class carriers have to have an extra mast aft of the island for some of their electronics.
I'd also wonder about the effects on deck turbulence. Might be able to reduce it with two islands or if it's not properly modeled make it worse.
You gain space for electronics, but lose deck space for aircraft staging. Only time will tell which is more important.
I'm not an expert, but I would think one large object is probably the most "attractive" to radar. Incidentally this is not the first time the Royal Navy has contemplated putting two islands on a carrier. When they were first developing carriers during WWI, there was a proposal to give HMS Eagle two islands - one on each side of the flight deck, connected by a bridge! The small aircraft of the time could have managed with the restricted clearance and short flying-off space forward of the islands, but the ship would not have been able to accommodate growth and would probably have had to be reconstructed. However the idea was not implemented; Eagle and her one starboard side island served until sunk in 1942. Eagle was built on the hull of a battleship under construction for Chile and planned to be named Almirante Cochrane. Her sister Almirante Latorre was close to completion when WWI broke out, served in the RN as HMS Canada, fought at Jutland, and was returned to Chile after the war. After Pearl Harbor, there was some thought of acquiring her for the US Navy, which would have made for quite a career!
Turns out the Italian LHD now under construction also has dual islands and a gas turbine/diesel propulsion plant. However it appears to be a conventional CODAG arrangement with prime movers connected directly to propeller shafts. She also has small electric motors which can propel the ship at 10 knots, seems like an emergency get-home capability. Fincantieri Starts Working on Italian Navy Future Landing Helicopter Dock LHD
With modern technology, I think its pretty obvious when you detect a Carrier. They aren't exactly 'stealthy'. And with modern weaponry, I would imagine the damage would be the same. Is there any advantage to two islands? Or is it just a design 'flaw/necessity'? Given the US supremacy in the modern era in regards to Carriers, I would have expected most countries to mimic the US in their design (if not on a smaller scale).
As noted earlier, it's largely due to the uptakes for the gas turbines. The Italian ships have the main engineering spaces separated by about 100', with the uptakes going straight up from each so they do not consume any more interior volume than necessary. We can see this in this photo of their existing carrier Cavour, although in her case both funnels are built into a single large island. The new ship basically omits the part of the island between the funnels. Italian aircraft carrier Cavour - Wikipedia
Ran across this diagram of the QE's engineering plant, showing gas turbine and diesel generators, propulsion motors, and propeller shafts. The staggering of the motors suggests that they are in adjacent machinery rooms separated by a transverse bulkhead. https://www.rolls-royce.com/~/media...uments/news/6-page-qe-booklet-tcm92-58802.pdf
Other onlin schematics show several compartments between the two diesel spaces. While the diagram does not show it well, the gas turbines are located in the island structures.
