No No NO!
My ex bf left me for someone else...and now I know how it would feel being on the other side.
If someone is in a relationship when you meet them, and they are sincere about ending it with their bf/gf and start dating you...they will end it with them asap. I think any guy or girl who cheats ....well I dont think very highly of them. If you are unhappy, leave...dont start hooking up with someone else until you get the balls to end your current relationship