Well, first and foremost you need to ask yourself if you love the person you're going out with at the moment. If not, then you need to leave him. It would be more or less like cheating if you still hold on to him. Then the answer for your question is clear :) You can maybe flirt with the guy you love, and get to know each other more.
If you love both the guys -- then it's a tricky situation. You just need to think who you love more (think the person who you cannot imagine your life without), and you have your answer!