How to set up wp-config with two usernames?

Hi,

I am setting up a joint blog with a colleague; we are wanting to be able to both add to and edit the blog. Do we set this up with two usernames in wp-config (and if so, how do I do this because the documentation only covers using one), or do we set it up with one initially and then somewhere later we add another user/author with all privileges? I have already set up the database with two usernames and all privileges assigned to each.

TIA!