Board Thread:Warhammer 40k General Discussion/@comment-6078851-20130130174042/@comment-5846164-20130213075949

horus brought the twin primarchs to the Emperor so i think he knew it was 2 of them.

Alpharius Omegon was eventually rediscovered by the Primarch Horus and sent to meet his father the Emperor on Terra, who placed him (and secretly, his brother) in command of the XX Space Marine Legion, the Alpha Legion, during the Great Crusade of the 31st Millennium.

hm here in the wiki, it says it in a way that makes it seem that horus only sent 1 of them, but they had no legion to hide in so either his brother was super stealthy to hide from horus knowledge, or so did he know there was 2.