Ideally, you want one list that covers both. And you want it tightly integrated with your email (both personal and work), calendar (both personal and work), social media, etc. But that means your work information begins leaking into the personal area, which presents data protection and commercial confidentiality issues.
A concrete example: say you are a manager who has an employee who is underperforming and subject to performance review. That performance review is going to appear as a to do item, and if that to do list appears in your personal sphere, that puts confidentiality at risk.
How can we bring everything together, but keep it apart?