Yes, it is legal to bet on multiple outcomes of an event or game. What is the risk? There is minimal risk involved when betting both sides. A user could make an error when placing a wager or the odds could potentially change while making wagers (try to avoid live games).
yes, but it depends on the circumstances. 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).