The same thing happened to me, with Yosemite second monitor won’t work. In fact, control panel will only recognize the presence of an additional display when I turn the monitor off. A bit whacky. Primary monitor is a Samsung 32 inch TV and secondary monitor is a Seiki 50 inch TV.
Bill
Mac Pro Early 2009
52 g RAM
1 TB Samsung boot with Apricorn
Multiple NAS