User blog comment:Supahbadmarine/By Tzeentch! Here We Go!/@comment-2253300-20130212214731/@comment-2253300-20130212222131

Yes, but they were isolated from each other at first, and raised on respective worlds. They didn't know each other's locations, as the Emperor wanted to limit the chance of them being targetted. They discovered each other due to their respective campaigns of expansion and conquest.