In regards to the other posts, we still don't know enough of A and O's thought process. We know enough to suggest they want to see Horus succeed, but maybe they have decided that there is another route. Or maybe the Kabal lied to them, or maybe Chaos has managed to take its hold in some members of the legion, maybe even one of the Primarchs and it sends the whole plan to ****. I just don't think we have heard the last of the Twin Primarchs. It's too good of a plot tool to just thrown into the heresy without using it in a great story or twist. Apart from the "oooooo" factor of them being twins, there hasn't been much point in that little surprise just yet. I just think we will see them at each others throats in time! (I hope!)
I don't even want them to regret turning traitor, I just like the idea of one of them seriously disapproving of the others methods. One falling to chaos, the other trying to heed the Kabal or whatever.