One option is to install Raspbian and then use the instruction on this site
to install XBMC. It's not a dedicated XBMC distro so it might lack some minor bells and whistles that something like Raspbmc or Openelec might have, but it's a very good setup. Then you could also install RetroPi or emulationstation for the emulator side of life.
Launch whichever program you wanted from the bash and you wouldn't have to worry about multiple partitions.
Conversely, yes, you could install NOOBs and install Raspbian and Raspbmc/Openelec on the same sd card using two partitions