This post hosts a thesis by James Mistry, Department of Computer Science, University of York, that documents a version of FreeRTOS that is able to schedule tasks across multiple cores.  Source code is also available from https://freertosxcore.svn.sourceforge.net/svnroot/freertosxcore (see http://freertosxcore.svn.sourceforge.net/ to download a tar ball).

James notes, at the time that this post was made at least: 

Do bear in mind that this is not production code and requires significant further
development before it can be used in a live product. Having said this, if you
are interested in a multicore version of FreeRTOS you should hopefully find
it useful!

Great work James!