There is an app available for the Samsung Galaxy S2 called SGS2 Dual-Boot Setup (LolBoot) that allows you to install two ROM’s on your device. LolBoot will allow you to switch between these two ROM’s whenever you want and can be found on the Android Market. With the over 25 ROM’s available on the Samsung Galaxy S2 already this LolBoot app should prove very useful. Imagine being able to switch between your stock device and CyanogenMod whenever you want, nice right.
How to Dual Boot the Galaxy S2 i9100 with LolBoot: