If I were you, I'd take my time on dating exclusively. They both know you're casually dating other people so just take your time and get to know them better. What's the rush?
And one more thing, I don't know about going back to the ex thing - if you decided on that. He's an ex for a reason and well, you were really hurt by his accusation. If you got back together, he'd probably accuse you of other things when things don't go his way.