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).
It's not illegal but bookies don't allow this usually as you get higher chances of winning this way. Or the ones accept it just lowers your limits or odds. Your best option would be to use different bookmakers for such things but they still have ways to detect those even.