Many a traveler has been locked in a silent battle with their seatmate over who gets the shared armrest. The best etiquette is to assume that every passenger gets to claim one armrest for themselves. The one that has your headphone jack and USB outlets is the one you should use.
Better yet, have a quick conversation with the person sitting beside you and make a compromise. If you can both agree to rest your elbows rather than your arms, then you'll both get two armrests.