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I internalized my halyards on a Endeavour37. Biggest problem I saw with external halyards is if you lose one at sea, it goes to the top and there you are.
I actually was able to use the 4 masthead pullys for double main and double jib internal halyards. They exit about 6 feet or so above the deck through an "exit plate". You need to stagger the exit plates so that the slots in the mast do not line up to weaken the mast. I had my mast pulled and the work done by a professional rigger. I use clutches on each line so I can use the winches with either halyard on each side. The extra is great for backup such as when going aloft or in case one breaks, a spare is ready.