Using dual booting, the dev team starts off with a jailbroken system. From there, another partition is created and iTunes is used to load the second iPhone OS. From the jailbroken OS, the necessary hacks are performed to the stock one and leaves the new version jailbroken.
As simple of a concept as it sounds, the steps necessary to complete the dual-boot are quite complicated. I have tried it and we don't recommend it to the novice users. I am in no rush to make a tutorial on this because I feel if you are smart enough to need the ability to dual boot, then you should be smart enough to figure out the already detailed instructions at the Dev Team's site. Read the instructions here.