What works for you will depend upon a number of things. If you're changing headsails often (racing comes to mind), two separate sheets make sense because you'll be able to take the windward sheet off the sail you have set and use it for the working sheet on the jib you're switching to. Then you lower the original sail, take the second sheet off it and tie the second sheet onto the newly hoisted jib for the windward side. If you only have one long, doubled sheet on the first sail, you'd need another long, doubled sheet for the second sail as well. This would be twice as expensive. Doubled sheets can wear unevenly, perhaps chafing on a long one-tack trip. If the damage is bad enough you then have to replace the whole double-length sheet. If you use two separate lines, you would only need to replace the one that was worn: half the cost. When boats are smaller, the cost differential in buying separate or doubled sheets may not be much, but when the prices go over $3.50/ft for heftier line, and you need 40' of it for a single sheet, 80' for a double, it can get pricey very quickly.