That's definitely the huge challenge with managing two systems. David Allen uses one system only. I used one for years as well, but in the past year moved to separate systems for work and personal. Since my work calendar doesn't sync to a mobile device, I am hampered trying to book some personal appointments (which does sync to my iPhone) without knowing my work schedule. There haven't been too many collisions, but it does occasionally happen where I book something personally like a dentist appointment, when I'm at the dentist's office, and come back to my work computer to realize it's in the middle of webinar I'm presenting. Best workaround I've found is for any short-term appointments personally or professionally, that are the most likely to collide, I wait to book those when I'm back at my laptop and have access to both calendars.
Some colleagues of mine who also have two systems carry both a BlackBerry for work calendar and a separate mobile device for personal. I didn't want a BB, so I opted to go without having access to that calendar.
Might be more than you wanted to know, but hope it helps!
Kelly