Well, part of the point here is the "too far away from their own capital not to revolt almost immediately", I think...
Secessions, as a rule, are allowed. If you intention is to create a new realm, then secession is the way to go.
A strategic secession refers to an event where it is not the intention of the characters to create two separate realms, but the players feel they have to do so in order to have two recruitment points.
If the seceded lands are too far to be held by the parent realm, then it becomes impossible to argue that the parent realm would have preferred to remain a single realm but split only in order to gain an addition recruitment point. If they had not split, then they would not have held to the lands they just conquered, rendering the conquest moot.
