Whenever you sacrifice a black creature, you may pay 2 life. If you do, each other player sacrifices a creature.
Whenever you sacrifice a green creature, you may gain 2 life.
Sacrificing a creature that’s both black and green will make both abilities trigger. You may put them on the stack in whichever order you want.
Savra itself doesn’t let you sacrifice creatures, and you can’t sacrifice them spontaneously. You must have another spell or ability that lets you sacrifice creatures.
These abilities trigger specifically when you “sacrifice” a creature. They won’t trigger when a creature is put into your graveyard for any other reason.