If there are two guys, one guy working on server-side code and another guy working on client-side code, either side could get held up waiting for the other guy to finish his part of the project.
Even if people aren't waiting around for something to be completed by someone else there is an overhead in communication. "Oh, you wanted the LIs to have that class name? Well I've got to go fix my CSS now... gimme a few minutes."
Even if people aren't waiting around for something to be completed by someone else there is an overhead in communication. "Oh, you wanted the LIs to have that class name? Well I've got to go fix my CSS now... gimme a few minutes."