X2 boss drop elixer question

[Thexkid]

So i figured the x2 boss drop elixer would work for rune farming and also xp farming in the dream map, which is really cool they made the elixer come more into play. I tried rune farming and the first boss kill, it dropped 2runes(1mag, 1vic) figured it must work.. but then tried 5 more runs after the initial and the boss didnt drop 2runes again, only one. Not sure if its just a chance to double drop something that the boss drops(fragments) or if its supposed to be guaranteed for everything he drops? Am i getting extra fragments when not looting 2 runes? (Doesnt seem like its working as intended)

-Boa