This is a personal choice thing and not a black/white rule with GTD.
If you do decide to blend them, it's not so much that they relate to each other, it's more about having a comprehensive system that allows you to toggle between whatever you choose to focus on, easily.
For what it's worth, I have:
Computer - Personal
Projects - Work
Projects - Personal
Someday - Work
All other lists are blended personal/professional, which works fine for me. All kept in the same system though of Lotus Notes.