Agreed. You don't want to share an entire system, but sharing projects is GREAT if both parties organize their lives via GTD.
Many of the online systems support some form of sharing. Being a Nozbe fan, I can elaborate a bit on that. You can share projects and both can see the task list for the project, but you can each assign your own contexts to items and you each can choose which are considered your own Next Actions.
In addition, tasks can simply be in the list or they can be labeled as assigned to someone to make it clear who is doing what. Even better, if you assign an action to someone else, it automatically hits their Next Actions list so they're aware of it (and which time they can un-Next Action it if they want).
It works quite well and supports as many users per project as you want. I've had some with three of us on it. Any more than that could get really messy, I think.