That's not really what probability says. If you have a 70% chance to win a particular game, your chance to win or lose the next game doesn't change. you still have 70% chance to win the next game.

what does change is the probability that you'll lose at least once. so, over two games, your chance to lose one game is 1 minus the probability of winning both games, which is 1 - 0.7 * 0.7 = 1 - .49 = 51%

the ultimate calculation as to whether a bet is worthwhile is Expected Value, which is your potential payout times probability of winning, minus potential loss times probability of losing. if expected value is positive, the bet is worthwhile.