Well we could either be spread out, or I was thinking even have chapters. So everyone has say four characters, and it goes like this.

Chapter 1: King's landing. Everyone's king landish character plays, action (whatever it may be at that time) ends.
Chaper 2: Dorne. Everyone's dornish character plays, action (whatever it may be at that time) ends.
Chapter 3: Iron Islands. Everyone's Ironborn character plays, action (whatever it may be at that time) ends.

And so on. Either is fine really, but being spread out would require loads of characters and incoherence between posts from different locations.