I wish we could use multiple build controllers on the same machine because we have many team project collections but they're not used at the same time so there's no real performance issue here and most of our build servers are not used for extended period of time. I know it's possible to do this with TFS 2010 and TFS 2012 (see following post) but it's an unsupported feature.
Anyone can tell me if it's now officially supported with TFS 2013, I've not been able to find any useful information about this.