Region wise driver assignment

Hi,
my client requirement is like i need to put constraint on driver region wise...
example :for region A:(only X&Y driver)
for region B(only P&Q driver)

I have itinerary based on region..
I have define same region as driver work region..
few of locations are part of other region...
so when i check show option of driver, it shows me all of the driver for both region
example: Region A, Locations are: 2,33,8,7,4, Driver:X, Y
Region B, Locations are: 2, 3, 33, 8, 17, 27, Driver: P, Q
when I check driver show option for a shipment 2 to 4 it shows me all the drivers: X, Y, P, Q
but when i check driver show option for a shipment 2 to 27 it shows only Region B driverS: P, Q.

can you plz suggest me if there is any other way to solve this issue...

Thanks & Regards
Sanjita