One of the subsets of string theory - M-theory - talks about M-branes, which are esentially paralel universes sliding across each other - quite akin to the various splinters in your game. There isn't really any need to justify the setting by this, its just a neat paralel. (Though talking about magic in terms of vibrating the strings of creation would be that much closer...)
Speaking of theorehtical phyiscs - do the various splinters act as fairly close mirrors of each other - or can the gravity and other constants vary in each?
If you can tell me about what type of technology you want, I can probably provide some assitance with the writing.
Check out this list of - you don't so much need to require timing as an interesting enviroment for the players to exploint.