Wouldn't do it. As they say, never sht where you eat:
1. As Monetary said, if things don't go right, that can be a hell of a mess to deal with. Who wants to be at work all day with the ex they just wish would go away, or even worse, the ex who just wishes you would go away?
2. That means that you will spend nearly ALL of your time with your significant other. Seems like it would be cool for a while, but then it would be like, "dayum, can a brutha breathe??" Plus, what if every time you see her/him, you want to rip her clothes off? But you can't.
3. Suppose you manage to keep it hush. What if you two get picked to work on competing projects? Or if one gets promoted and the other doesn't? What if you just have stark differences about how to do the job?One of you gets praise from the boss, the other gets told his work isn't up to snuff....hurt feelings?
4. What if it's a hush hush relationship, maybe one person has a bf/gf or spouse, but the other falls in love, so you end it. It was supposed to just be a fling, but now they've caught feelings and they're hurt, so they decide to get you back. And you know, people who sleep together tell each other all sorts of little secrets...pillow talk is a mf.