Case: A PPC management firm with two clients that sell the same product.
A thread at SEW named Ethical Issues of PPC Management for Multiple Clients in the Same Field discusses one members ethical dilemma with this situation. Of course, this makes for a very interesting debate in the forums, a thread I fell is worth checking out.
If you have two or more individuals in the company that do the hands on PPC management, then I feel the solution is simple. Have one individual manage the first client and the next manage the second client. This way the same individual is not competing against himself.