[Galaxy S II Q&A, Help & Troubleshooting] Phistachio:
Hello fellas :)

This guide is just a quick how-to on dual booting. Many people have doubts on how to dual boot on Siyah, because it's something new and perhaps "scary" because they may think it can brick their phone.

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Requirements

A Galaxy S II I9100

3.x Siyah Kernel (3.2.x preferred). For latest version, visit the Siyah Kernel thread

90% charged battery

2.5GB of free space in your internal SD card

Patience

Time

So, do you complete all those requirements? Time to go to action. BUT BEFORE, a brief FAQ in a simplified way, based from this post made by the amazing droidphile :)

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FAQ

Can I dual boot Ice Cream Sandwich and Jelly Bean?

Yes you can, using Siyah v3.5.2.

1) What is dual booting?

Dual booting is the ability to boot into two different Ice Cream Sandwich-based ROMs (like CM9 and AOKP).

2) How is this done?

This is achieved by sharing the kernel and using a hidden partition in dev/block to store the 2nd ROM's /system and the /sdcard (Internal storage) to store /data and /cache inside a hidden folder (/sdcard/.secondrom)

3) How much does it take to clone a ROM?

It should take around 7 minutes

4) I want to dual boot fasturr. wat do?

When you get the splash screen to choose between ROMs, touch the screen or press volume UP to enter the PRIMARY ROM, and press the Home button or volume down for the SECONDARY ROM.

5) How can I remove the Dual boot and its partitions?

Just use the "Remove 2nd ROM" option in the Dual Boot options menu.

6) If I format my /sdcard, will my second ROM go kaput?

Yep, because you're wiping the folder that contains /data and /cache.

7) Can I dual boot a gingerbread ROM in a ICS ROM?

Niet. No. :p

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Now, the PROCEDURE

You must flash the Siyah kernel, either with CWM or ODIN

After you have flashed the kernel, reboot and let Android upgrade itself.

All done? Alright, move up to next step!

Download the desired ROM that you want to make it secondary, and place it in a safe place in your internal SD card

After that, reboot to CWM Recovery, and navigate to "Dual Boot options"

You'll see a list of options, namely :

View current status

Clone 1st ROM to 2nd ROM

Clone 2nd ROM to 1st ROM

Swap 1st ROM with 2nd ROM

Swap 2nd ROM with 1st ROM

Remove 2nd ROM

Install 2nd ROM from Internal SD card

Install 2nd ROM from External SD card

Wipe 2nd ROM data/cache

Wipe 2nd ROM dalvik-cache

Fix 2nd ROM permissions

What does each one mean, you ask?

View current status - This option will show you the stats of the dual booted ROM.

Clone 1st ROM to 2nd ROM - This option will copy (clone) the current ROM that you use (for example, CM9) and paste it to the 2nd ROM directory. Takes around 7-20 mins.

Clone 2nd ROM to 1st ROM - This options copies the 2nd ROM (after installed of course) and pastes it to the place where the 1st ROM was, replacing it.

Swap 1st ROM with 2nd ROM - This will swap the 1st ROM with the 2nd ROM, making the Primary ROM -> Secondary ROM and the Secondary ROM -> Primary

Swap 2nd ROM with 1st ROM - This will do the exact opposite of the above option

Remove 2nd ROM - This will completely wipe and remove any 2nd ROM files and folders

Install 2nd ROM from Internal SD card - This option is to flash the ROM to the 2nd ROM directory from the Internal SD card

Install 2nd ROM from External SD card - This option is to flash the ROM to the 2nd ROm directory from the External SD card

Wipe 2nd ROM data/cache - This will wipe the 2nd ROM data and cache. When it's done for the first time, it'll create data.img and cache.img and create the folders inside /sdcard/.secondrom, preparing your phone for the 2nd ROM.

Wipe 2nd ROM dalvik-cache - This will wipe the dalvik cache of the 2nd ROM

Fix 2nd ROM permissions - This will fix the permissions of the 2nd ROM

Select the option Wipe 2nd ROM data/cache. This will prepare your phone to flash the 2nd ROM. It takes around 5-7 minutes.

After having that done, select the option to wipe the dalvik cache.

After all that is done, select either "Install 2nd ROM from Internal SD card" or "Install 2nd ROM from External SD card" depending on the location of your ROM

Wait a few minutes for the flash

Reboot

Press the Home button or volume down to enter the Secondary ROM

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After you have done all that, you're set and you're in the Dual Booting world! :) I hope that this guide helped you, and if it did, press the thanks button!

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Credits

GokhanMoral - For his amazing kernel

Droidphile - For his great FAQ