Multi-Hand Blackjack is offered in online casinos. It allows a player to play from one up to five hands per round. You should play every hand according to the basic playing strategy. Your theoretical loss per hour will decrease if you take what you would have bet on one hand and spread it to two or more hands.
You should play multiple hands only if you can play at a reasonably quick pace. Other players will be annoyed if you spend an inordinate amount of time deliberating over each of two (or more) hands. You should play multiple hands only on adjacent spots.