I think the best approach is to let them know before you get involved with them. She could be getting herself into something she isn't prepare for. You can't be so selfish that you'll use someone else for your own benefit at their expense. Make sure the air clear.

I also would suggest you letting your wife know that you still have interest in being with other women. It'd save you a lot of the paranoia and sneaking around. I personally have no interest in stressing myself out unnecessarily. You'll be surprised with what a woman is willing to accept if you approach her confidently and honestly.