There's no absolute starting point, but all things being equal, the starting point might be half way in for both, so you have the most room for adjustment. Following that logic, if you need to increase the tension on the side where the screw is already in most of the way, then you should loosen the opposite screw.

That answered the second question, tightening one spring has the same effect as loosening the opposite. Imagine a seasaw with the kid at one end heavier than the one at the other. To get the seesaw level, you can push down the light kid's end, or lift the heavy kid's. Likewise for centering V-brakes.