Betting on both teams (also called arbitrage betting or middling) can result in the bettor making a profit regardless of the outcome by placing one bet per each outcome (and with different betting companies). Again, the conditions must allow it.
Bettors are far less likely to win parlay bets than individual bets, and sportsbooks make more money from parlay bets than individual bets. The odds on individual bets are calculated even to generate a small profit for the sportsbook. That gives gamblers a 50% chance of making a profit with standard bets.