I see from the published stuff that if you elect to be a DM you cannot also be a player - which seems fair enough as you would have an unfair advantage.
However, there is a possible way round it.
In the PnP days I ran a campaiign with about 20 players and 3 DMs - each of whom also played. The ' World' was split into 3 continents seperated by impenetrable oceans/mountains/deserts.
No one could play on their own continent but could play on someone elses.
Maybe when NWN gets big enough the rule could be relaxed?