Ok, I read this fic, so I know it exists! I just can't find it now, which vexes me greatly!

Sam has the ability to astral project and he messes up somehow and becomes disconnected with his body. To all appearances, he's in a coma. Jess calls Dean for help. Dean comes, and Sam possesses both Dean and Jess individually in order to interact with the other. He also uses their bodies to, um, have congress with the other. Eventually they get Sam back in his body.

Thanks!

ETA: Found!