How to dual boot your custom ROMs
Keep more than one ROM installed on your device at any one time.
If you’re a fan of custom ROMs you will have no doubt at some point wished you would be able to keep more than one installed on you device together. Maybe you want to try follow the progress of a ROM that isn’t yet ready for full-time use; or to have two vanilla and skinned versions of the OS running together; or perhaps you are just of the incessant back up/wipe/restore cycle that is an integral part of the custom ROM world.
With the Siyah Kernel it is possible to dual-boot certain ROMs on Samsung devices including the S II and S III. It only works with ICS and Jelly Bean ROMS, but supports popular ones such as those based on TouchWIz, CyanogenMod and MIUI. The main action takes place in Recovery, where your existing ROM is set as the primary ROM and then use the “Install zip to 2nd ROM from SD card” option to flash a secondary ROM in exactly the same way you would flash one normally. The phone will boot to the primary ROM by default. To boot into the secondary ROM hit the home/volume down keys within three seconds of turning on the device.