Just to point this to the right direction:
I use a dual boot setup with raspbmc and raspbian on one 16 gb sdcard
to do so
1) use NOOBS or NOOBS-lite from here
and follow the install instructions
choose both OSes, install and configure them.
2) install retropie/emulationstation on raspbian
Now you can make a choice between xbmc and emulationstation at startup
from here on it’s optional:
3) get the advanced launcher addon for xbmc (use google for the repository)
4) find out which partitions your OSes are on and write two scripts that edit the noobs config file to boot to a specific partition
5) tell advanced launcher to launch your script to boot to raspbian (add sudo reboot to this script if you want to do it instantly)
6) for the moment i added my script to boot to xbmc next time to raspbian’s init.d, meaning it runs on startup. Can i configure emulationstation to run a script? this would be the best solution I think.