Two blogs, same users, different capabilities

I’m trying to setup two blogs on the same database, which will share the same users who might have different capabilities.

I’ve thought of two ways:

a) Same db_users table, different *_usermeta tables.
b) Same db_users table, same db_usermeta table but different *_capabilities and *_user_level meta_key’s.

Both ways work great *if* they are manually adjusted. However, when a new user is created, it gets capabilities for only one of the blogs, and can do nothing on the other.

I will have to use the Role Manager plugin on both of the blogs, so I think “option (a)” will be a better way of keeping databases clean.

If someone can help out with this, it will be really appreciated.
Thanks in advance.

EDIT: I’m thinking of adding a WordPress mu installation to the same userbase, so if you have any warnings, I would appreciate them too.