link the second turret to the tank in the same way that the default turret is linked, with 'select and link' tool
In the default conditionstate, you hidesubobject the second turret, and in the weaponset upgrade conditionstate, you hidesuboject the default turret and showsubobject the second turret.
If the new turret has the end of its muzzle in a different place or more barrels etc, then you can also define the new weaponlaunch bones etc in the condtionstates.