Oh yes...
I actually lost a great guy because I couldn't let another guy go. I kept having these feeling for the first guy, and # two simply couldn't take it anymore, and after almost a year of giving it his best shot, he gave up on me. This was a actually a wake up call for me, because the guy I was hankering for was totally wrong for me.
So yeah, it is possible, but I would force myself to make a choice. Otherwise you're not being fair to your man or to your self. (And if your ex is in on this, he's probably not too happy about it either)