Yes, this is possible if you have stock on internal memory and CM on SD. If you use bokbokan's version of CM10.1 or CM10.2 on SD it comes with a boot menu that allows you to choose. If you are using verygreen's version of CM10.1 or CM10.2 on SD, his does not have that menu. But if you look at my HD/HD+ CWM thread linked in my signature, I have made a zip to add that menu to his version. See item 9.
Sent from my Nook HD+ running CM10.1 on emmc.
See my Nook Color Tips Thread here
and my Nook Color EMMC Partition Repair Thread here
and my Nook Color Updated CM SD Installation Instructions here
and my Nook HD/HD+ stock tips thread here
and my Nook HD/HD+ CWM Root Tools thread here