Business development is difficult stuff. It takes full immersion of your life. Strategic thinking, internal role playing, constant activity and thorough follow up are all necessary components of a successful career in sales.
I have known people that have tried to have two pursuits at once. Perhaps a job in corporate sales and selling real estate on the side. They are effectively serving two masters. Even the greatest sales people can only be good in this scenario. Also, the resentment you will have for one or both pursuits is inevitable. When you know you are capable of greatness but aren't achieving it, it leaves you feeling empty and this will make you resent your efforts.
Greatness requires a full commitment to a single "master." Unless you're Bo Jackson, in which case the rules, even if derived from scripture, don't apply.
For the past 5 years I have been serving two masters. Soon this will change and I'm very excited to see what I'm capable of with a singular pursuit. It's been years since I've had this luxury.
So, go forth, don't spread yourself too thin and give yourself to one professional pursuit! Let it be something you enjoy intensely.
By day I'm working for a large company, by night (and during breaks) I'm involved with two radio stations and some national efforts on community radio (see today's Pubski for more info). That's very much two masters...three, even. However, building support networks and coordinating these efforts with others has greatly reduced the stress. You're right on with what I've quoted though, it keeps feeling like imagine what I could do if I could do this for 40 hours a week and have a focus on it...could be greatness, indeed. But now is not the time to make that change - we'll see in a couple of weeks, pending a program I'm planning on applying to.