@liz7 lets say your MH primary attack does 100 points of damage. with multi attack +2, you get 2 more free attacks with your MH primary attack, but they do only 50% damage each of the original attack. lets say you are using a staff. it's primary attack is Comet. the Comet does 100 points of damage, with multi attack +2, when you shoot the Comet, 2 more Comets will shoot out too, but they both will only do 50 points of damage each. if all 3 Comets hit the same target, then the target will get a total of 200 points of damage.
Orb is a Special attack and doesn't benefit from multi attack. there is a Mythic affix called Discordance. what it does is switch the Primary and Special attack of a MH weapon. so if you had a Wizard Sword, and used Discordance, the Primary attack would become Orb, and the Special attack would become Pierce. multi attack would then work on Orb, but not on Pierce, because Pierce is now considered a Special skill, not the Primary.