Full Idea
Nothing can be in two places at once. If other possible worlds are really other universes, then clearly, you and I cannot be in them if we are here in this one.
Gist of Idea
If possible worlds really differ, I can't be in more than one at a time
Source
Robert C. Stalnaker (Counterparts and Identity [1987], 2)
Book Reference
Stalnaker,Robert C.: 'Ways a World Might Be' [OUP 2003], p.114
A Reaction
This can be sensibly expressed without possible worlds. I can't embody my other possibilities while I am embodying this one (I'm too busy). Insofar as possible worlds are a good framework, they are just a precise map of common sense.
Related Idea
Idea 16408 Rigid designation seems to presuppose that differing worlds contain the same individuals [Stalnaker]