You might ask "why would I want to do this?", this is a great option if you want to use two different ROM's like CynaogenMOD9 and a Samsung based one or maybe a MIUI ROM. This is also a great option if you just want to test out a different ROM without needing to do a full backup wipe and restore every time you want to switch between ROMs.
Here is an example of how it works on my phone (sorry for the video quality, also I figured the music was better to listen to than the ambient noise)
Before you continue please check this FAQ for information and supported ROM's
to enable this feature:
- Reboot into recovery
- select "mounts and storage"
- format /secondrom_cache
- format /secondrom_data (this may take up to 10 min so just wait)
- format /second_rom_system
- go back
- dual boot options
- install ZIP to 2ndROM from (where your ZIP file is for the ROM)
- when prompted select volume up for the primary ROM and volume down for the secondary ROM
There are a lot more options in these menu's please see the kernel thread for more information and support related questions.