I saw this mod on Skyrim Nexus a while ago but before I could download it the webpage says the author made the file hidden. This mod is suppose to allow up to two companions, and the only way to get rid of second companion is to hit him/her repeatedly until they get hostile lol. The mod was located at: http://www.skyrimnexus.com/downloads/file.php?id=3021

There's also another companion mod out there but only lets you get companions as part of a quest.