Oh...you better sit down for this one.
So I dated my ex, for what I recall to be 2 years. Met at work, we started dating, everyone saw how well we got along, always asked if there was something going on. So for a year or so, we managed to deny everything. We had broken up within that time, and let me tell you...working with your ex isn't the most pleasant thing in the world. (Cue the "don't shit where you sleep" comment, you know you wanna).
We got back together...proceeded to date....he gave me a ring, blah blah. We finally announced it to the coworkers, when I left the company. And of course everyone did the whole "aah we knew it!" thing. Fast forward few months and I call everything off, ring returned and there's a job opening at my new place. After getting over the "i hate you" phase, I told him he should apply for the role (it's a great company & great opportunity). He does, gets the job, we are on good terms, even carpool to work together. That was nearly 3 years ago. We work fantastic together, he's a great guy, terrible boyfriend. Don't get me wrong, it didn't just miraculously happen. There were many team meetings where I wish a laser from space would take him out. There were many unhappy facial expressions towards him. But after a while, I realized.....well crap, I work with him, better make this as comfortable as possible. We now sit, literally, 3 or 4 feet away from each other. I confide in him and he does the same, it gives you someone you can trust. I still do make comments towards him, because...well...I'm the ex, and I can do that. I like to remind him of the things he never did, and take stabs at him when I get a chance, cuz its fun, but all jokingly. He'll tell me his relationship/dating stuff, I do the same. We are clearly better off this way than being together. But IT CAN be done. It really depends on your personality. Neither of us are the jealous type, so hearing about the other person's dating life doesn't bother us. Again, can be done. Takes some time, allow yourself to get over some stuff. My situation was a little different, as we were forced to deal with each other due to work. We never had the opportunity to do the NC rule, it was literally impossible. It is a bit of a odd topic to bring up to someone new you're dating. Kinda have to casually throw it into a convo of...oh hey...btw....I work with my ex...see him more than I see you...how are your nachos?
And on that note, there are some people who are not ok with that. And that's ok. My last relationship....the guy mentioned that my work exbf was his "nemesis". Clearly, that's why he was the last relationship & not my current :)
Hope that helps!