Board Thread:Warhammer 40k General Discussion/@comment-108.20.1.54-20140516152133/@comment-82.47.209.198-20140516180425

I know for a  cert that the Ultramarines worked largely independantly because they were so large, and were led by none other than Roboute Guilliman, and that they were mostly engaged in the far eastern corner of the galaxy. I also know that the Iron hands and the Emperors Children worked closely with one another on numerous occasions, hence the close brotherhood between Ferrus Manus and Fulgrim, (until of course the latter was corrupted by chaos).

Not sure about many many of the others, but i know that Horus worked with each of his brothers legions on at least one occasion or more.