I don't really understand the way you explained the tank object but i would go about like this...
Make a tank body. Use the body as a base sprite.
Next, make the turret and pin this onto the tank body in the correct place and add the 'pin-to' behaviour and 'turret' behaviour.
Pin the turret to the tank but only set it to 'pin to position' not angle. This will mean that the turret will follow the tank body but can rotate at it's own accord.
This does mean that when you turn your tank you turret won't rotate but, if you want, you can sort this out by pinning the turret (position and angle) to the tank body when the tank is turning and then just pinning the turret (position only) when the tank is not turning!
Hope this helps!