Using a snatch block will double your available pulling power. It will also allow you to perform pulls at an angle to your truck.
When doubling up the winch line using the snatch block, where would you attach the return line?
When attaching to another vehicle for towing or pulling you would connect the return line to a third vehicle or tree spread the load.
If you are trying to free your own vehicle and you are using a tree to anchor then the return line should be hooked onto your vehicle's frame. You should never hook it back on to the winch mount as this will effectively double the load on the mounting plate.