1 System if Possible
I think that unless your work security restricts you to keeping work items separate it's a whole lot easier to integrate the GTD method into a single system.
After all you can't turn off your brain like a machine. How many times do you think of something that affects work when at home? and how many times do you think of something you need to do or be reminded of at home when you are at work? You at a minimum need some way to capture those thoughts and get them to the proper location for processing if you keep 2 systems and if you are just starting out that may mean it's an extra layer of work that causes you to avoid doing it at all.
Oogie McGuire - Mac, iPhone & Omnifocus
OogieM on Twitter
Paonia, CO USA