Who's the host and who's the co-host?

If you cringed when you read that topic name, then we're in agreement. I've heard people use "co-host," or co-anything for that matter, to refer to a lesser or secondary role, and it drives me a little crazy. It's like calling the VP the "co-President."

In fact there was an episode of Scrubs where J.D. (the main protagonist) and Elliot are both promoted to the position of Chief Resident. Part of the episode's sub-plot involved determining who was the Chief Resident (with a higher salary, of course) and who was the "Co-Chief Resident." At the end, J.D. agreed to take the lesser "Co-" title. Awww.

I've always known "co-" to refer to an equal partnership or joining, such as in words like "cooperation" and the concept we're discussing in class as I type this, "coarticulation." If there are two hosts of a certain logophiliac radio show, I would think of them as "co-hosts." In this case "host" as a title doesn't really apply, since that only refers to a single person filling that role, and the idea of having a "host" and a "co-host" seems paradoxical.

I'm fully prepared to accept this as something people are just going to do, but I want to make sure: is there any precedent beyond common usage that allows for "co-" referring to a subordinate or secondary role?