I've found a bunch of different articles that CLAIM to answer this question but they simply don't. Please don't refer me to an article that mysteriously does not address my question. I was finally able to find a work around to make the airplay icon appear in my iTunes (using iTunes 12 and Mac OS X Yosemite.) but it won't allow me to select multiple sources to play to. PLEASE HELP!
MacBook Pro (13-inch Late 2011), OS X Yosemite (10.10.2), frustrated!