If you try this, I would recommend deleting any previous CM9 *.img files from your multiboot/CM9 directory and rerunning the dualboot setup zip. Alternatively, you could move/rename the data img file if you'd like to restore your CM9 data to a future CM9 boot. It may work, but I would not recommend booting CM10 with CM9 data or vice versa.
NOTE: a slight modification to the cm10 update zip needs to be made until the fakecwm can be updated. This modification should not be done if you are not using the dual boot utility. The updater-script needs to be edited:
Replace the line:
format("ext4", "EMMC", "/dev/block/mmcblk0p14", "0", "/system");
format("ext4", "EMMC", "/dev/block/mmcblk0p14", "0");
1. Delete previous CM9 *.img files from multiboot/CM9.
2. Reflash TeamEpic-SGS3LTE-multiboot-cm9-setup-v1.zip to create fresh disk images.
3. Copy boot_CM10-v2.zip to your phone. EDIT: v2 added 08/12
4. Flash boot_Fakecwm.zip.
5. From within the Red colored Fakecwm, flash your modified cm10 update zip.
6. Optionally flash the jb gapps.
7. Flash boot_CM10-v2.zip.
Which CM10 do I flash? I tried the most recent offical (8/15) and I got status error 7. I have follow all the instructions for cm9, you just use a different file to boot right?
Edit: Had to edit the updater-script, git it working thanks to androidrhodium for helping me out a bit.
This is bbelos' Instruction from the CM10 Official Thread