Don't know how to solve this problem in a professional way, but from a user/low level programming skilled person view, an additional - lets call it tag-2 - layer (and field and whatever it needs to link/list tag-2 posts) seemed to do the job in my imagination.
Would that be complicated to implement in the next release?
One could use such a thing not only in a multi-author environment, but also if one runs a multi-language blog, which is one of our next projects and where i already see the challenge to offer a user-choice for his/her specific language.
Err... Do you realize there is a built in feature to list all posts from one author?