well afaik, your multistrikes dont have a chance to multistrike or you could in theory multistrike indefinately but rather your spells have 2 chances to multistrike which is why you can cast a spell and have 2 minor versions of it following it, thus having multistriked twice which afaik is the highest possible amount you can multistrike. if you read the description in your character tab (pressing c) and you put your mouse over multistrike, it clearly states that your spells have 2 chances to multistrike for 30% dmg(if i remember correctly)
its quite funny to see your average dmg on say chaos bolt being higher than your max hit, like having skada say your max hit was 68k and then have your average dmg at 75k due to multistriking.