How to Dual Boot Custom ROMs on Galaxy S3

For those of you who want to dual-boot 2 different custom ROMs on your Galaxy S3, the latest Siyah kernel supports dual-booting 2 different Jelly Bean ROMs.  ICS ROMs are NOT SUPPORTED but so long as you use Jelly Bean custom ROMs, you can dual-boot them.
Currently Siyah kernel is only available for international Galaxy S3 GT-i9300 models, please do not try this on your dual-core U.S./Canadian models!
Why dual-boot?
For example, CM10 Jelly Bean ROM is awesome ROM but you do lose a lot of Samsung features/apps.  By dual-booting, you can have best of both worlds, dual-boot CM10 and a Touchwiz-based ROM so you can easily switch back and forth.
Step 1. Before we begin, you need a rooted Galaxy S3 on a Jelly Bean ROM (stock or custom).  And also download another ROM you want to install onto your phone (2nd ROM).
Step 2. Next, we are going to install Siyah kernel using CWM Recovery.  (You can also install using ODIN if you want using the .tar file version here.)
Download the Siyah Kernel zip file and copy over to your phone.  Then turn off your phone.  Then hold down Volume Up, Center Home, and Power buttons together to enter CWM Recovery.
Download:

  Download Siyah Kernel zip file

 

• Step 3. Once in CWM Recovery, choose “install zip from sdcard”
• Step 4. Choose “choose zip from internal sdcard”.
• Step 5. Find and install the Siyah kernel zip file you downloaded earlier onto your phone.
• Step 6. Reboot and you will find a new app called “STweaks” installed on your phone.  Open the app and swipe to the right to “Dualboot” tab.  Choose “Reboot into 2ndROM Utils”.  This will get you into the Siyah CWM Recovery.
• Step 7. Once in Siyah CWM Recovery, choose “dual-boot options”.
• Step 8. You can install your 2nd ROM straight to internal/external SD card or even restore ROM from one of your ROM backups.
• But for purposes of this tutorial, we will clone our existing ROM into 2nd ROM so choose “Clone 1stROM to 2ndROM”.
• Step 9. Once done cloning, you can install a new custom ROM on your Galaxy S3 just as you would do normally.
• Step 10. Choose “wipe data/factory reset”.
• Step 11. Choose “install zip from sdcard” and install your new custom ROM.
• Step 12. After installing your new ROM, reboot.  If you don’t see an option to boot 2nd ROM during boot, that means the new ROM you installed does not come with Siyah kernel and you need to install it.  Repeat Step 3-5 to install Siyah kernel on your new ROM (1st ROM).
• Step 13. Once you have both custom ROMs running with Siyah kernel, you will see an option to boot to 2nd ROM during boot.
• Step 14. Anytime you want to boot 2nd ROM, you can hit the Center Home or Volume Down button during boot.

That should get you jump started dual-bootin’ your Galaxy S3.  Galaxy S3 again proves to be one of the best Android devices to dual-boot multiple ROMs.  Although there are other phones that can do it too, Siyah kernel makes it super easy even for your regular ol’ Joe.
If you like this, don’t forget to donate to the developer of Siyah kernel through the XDA credits link below.  Developers put a LOT of hard work and free time into their kernels/ROMs, if we all donate even a dollar, it goes long ways to more new cool stuff like this, thx!
Credits – XDA