I've been moved to a busines development part of the business but I'm still in charge of all of our SEO and SEM. I was only an advanced beginner to begin with but our needs have grown. I'm not sure whether to contract with one or two people. Can someone be extremely well versed in all things SEO and SEM or is it better to get two people on board that might serve as a better sounding board?

While they certainly work together, both are a constant moving target and it requires us to stay on top of trends and white hat policies of all the major players. My worst nightmare is to get someone that tries to game the system and screws up our rankings long term.