Advantages of dual ballast over single??

The only "adavantage" that I can think of for using a 4 prong ballast is if you really enjoy replacing them. Even when these cars were new, many people learned to carry a spare 4 prong in the glove box. When I was working in dealerships in the 70's, the service writers kept them in their desks and used them to plug into the tow-ins from the weekend on Monday morning so that we could clear the service drive to open.
After Chrysler went to the 4 prong ECU/2 prong ballast combo, about 1978, they got a lot more reliable. On my 75 W200 when it had the 4 prong, if the engine was warm and you opened the hood after a rain or washing the truck, water would drip on the hot ballast and kill it every time.