Board Thread:Warhammer 40k Novels/@comment-27376167-20151210113237/@comment-24091837-20160414000645

My guess is that one Primarch sympathised with the Tau philosphy of the Greater Good, & thus joined them, while the other Primarch went on to side with, & join the Eldar. They both turned away from the Imperial cult, due its belief in the superiority of mankind, and its antagonistic attitude towards all non-humans.