Here’s some great news for flasholics who want to run two ROMs side-by-side. With the Siyah ICS kernel, now you can dual-boot two different ROMs easily than ever before.
Basically your second ROM with get stored in secondary file system and the user will have the choice to boot into primary or secondary ROM.
This is probably a great way to test out two ROMs on the go, so give it a whirl and let me know how it works for you.
You will need ICS ROM and also Siyah kernel, you can see How to Install Kernel.
Also, use the latest Siyah to dual-boot ICS and Jelly Bean ROMs at the same time!