[Galaxy S III Q&A, Help & Troubleshooting] tisazalay:
Hi,

I'll just add my two cents, (remember to have nandroid backups before you do this)

I managed to dualboot efficiently using this method described earlier, what is required is Siyah Kernel (as far as I know the only one that supports dualboot), Temasek's CM10.1 based, Omega 35, CWM based recovery (I use CWM 6.0.1.2, but you need at some point the one with Siyah).

Why Temasek's? Because he disabled multiuser in his build that is mainly the problem with the 0/1/2 etc. folders and it's a superb ROM, you will love it once you use it, he also has a Kernel (with the Exynos fix...)..... Why Omega?, well if you need to ask then stop reading ASAP.

The clue is to first setup Omega as 1st ROM, this is simply done by installing it via CWM (v. 6.0.1.2, fully tested), you can boot it to see that it installed properly.

Then you need to install Siyah kernel (it's not included in Omega), if you don't find the Stweaks app (inside the app drawer) when you boot the phone, go to Play Store and install it.

From Stweaks you go to the dualboot tab and select the boot into recovery (not hijack).

Inside Siyah's recovery you now need to place Omega into 2nd ROM, how? Go to the last option into the dualboot menu, there you have several options but you need either swap (if you had a working 2nd rom) or CLONE 1st rom to 2nd rom. This process is lengthy, you wait, then to test you reboot and select 2nd ROM, it should work, if by any chance you don't know that you are in second rom, simply go to stweaks and from the dualboot tab select boot into second rom, then mate, you are in 2nd ROM.

Boot into recovery (not necessary siyah recovery, cwm 6.0.1.2 touch will do)

Install Temasek's CM10.1 based, (as a normal 1st rom install), One thing is very important, OBSERVE all the rom installation advisories, particularly to full wipe (format system, data, cache, dalvik cache, etc.) before you install cm10.1 (it's not samsung based). Then you install CM10.1 (another warning is the problem with 64gb cards formatted other than FAT32 that may have problems... so please, avoid other formats specially ExFAT).

Reboot the phone (should take you into CM10.1) and you configure it and play with it and then at some point you need to install Siyah again (cause it disappeared with CM10.1 install) from wherever you want CWM (is easier) or Odin if you are confident with .TAR...

When you have installed Siyah you should be able to select which ROM to boot and BOTH are working, Omega as 2nd and Temasek's as 1st.

You can merge other stuff like superwipe but I feel it's unnecessary if you do a full wipe before installing CM10.1

Good luck, and share your experience here.

NOTE: I had triple booted efficiently with MIUI (third/second), Omega (Second) and Temasek's (1st) but it requires the triple boot utility and better to first get dualboot working.

IF YOU DUAL(TRIPLE)BOOT ANY OF THIS ROMS AND KERNEL PLEASE THANK THEIR DEVELOPERS IN THEIR OPs, it shows your gratefulness and it's a small reward for them and their time. Of course if you can donate to them or their causes in their name that will encourage development.

Happy New Year Everyone