Dual booting Yosemite/Mavericks: Stuck in Yosemite with an 'Install OS X' volume instead
I also asked on the developer forums and got an answer there (iOS/OS X dev account required to view).

The solution was to boot into a recovery partition (⌘ + R), and change the boot disk from the Apple menu. It needed to be changed to my Mavericks install instead of my Yosemite install.