Two Tier Campaign

I've been reading Last Guardian of Everness lately, in which some of the characters take action not only in the real world, but also in the Dreaming. It occurred to me that having a two-tier campaign would be one way to give players some variety while maintaining campaign continuity. So, to give a D&D 4e example,  in the real world, you're playing low level characters, perhaps L3 to get started; in the Dreaming, you're significantly higher, perhaps L15, and possibly a different class and perhaps even a different race. In the real world, you're a L4 halfling rogue; in the Dreaming, you're a L16 warforged juggernaut, or eladrin warlord, or pixie sorcerer. And perhaps your Dream character changes from time to time, if you want--sorcerer for a few weeks, then paladin, but it's still you inside. You can get some variety, and there's no continuity problem of "We were in this dungeon, and Fred the Fabulous decided to walk off, and then around the next corner, behold, Phred the Pious  appears and we immediately trust him implicitly."