For the Happiness facility ability (both of them), only the best, most positive value is used. They DO NOT stack in any way. If you have a facility with -10 and one with +1, the planet will get a +1 bonus to happiness each turn. The -10 is essentially ignored. This sucks, but it is how it works. Unlike most abilities, the planet and system happiness ones seem interconnected in that they do not function independantly. Only the best of all facility happiness bonuses to affect a planet will be in effect. A -1 planet happiness and +1 system happiness facility will net in +1 happiness bonus for the planet, not 0. At least, this is how it seemed to work when I tested it a long time ago.
