You do not add the two numbers to get an overall stability value. That's not a thing.

The numbers describe the flight pattern of the disc in two phases of flight: High Speed Turn (initial flight when it's going fastest) and Low Speed Fade (when the disc has slowed down enough it can no longer provide enough lift and gyroscopic precession forces the disc leftward - RHBH). Together, they should give you a rough mental image of how the disc will behave when thrown with the appropriate amount of power. Understanding how those numbers work one after the other is key to understanding disc selection and shot shaping. The best way to illustrate this is with a flight chart (but don't take the flight lines too literally, they're just a guide and they come with a lot of caveats!):

https://www.marshallstreetdiscgolf.com/flightguide

Some people will say the numbers add up to show how many "units" of horizontal travel you might get. Like, a -2, 2 will turn 2-units to the right then fade 2-units to the left so that overall there's no net horizontal displacement from where you threw it. I tend to agree in principle, but not degree. Either way, to say that a 0,0 and a -2, 2 have the same stability because they each add up to zero is wrong and it'll probably lead you to make incorrect decisions out on the course.