If you plan to make the deck free-standing (with no ledger attached to the house), you obviously need 2 support beams. Doing so is the preferred method, as it eliminates the need for the ledger and its associated, long-term performance problems. Locating the joists with 2' cantilevers at each end will work well, resulting in a balanced design with minimal deflections if you space the 2 x 10 joists at 16" centers. If you're only using 3 posts (6 x 6s recommended by the IRC), I'd suggest cantilevering both ends of each beam to shorten the continuous spans to 8' each. Less "wiggle" and deflections when you hold those big parties on the deck.