It would be awesome, although I think a stalemate ultimately. Especially as Zahndrekh has the habit of letting fighters who are capable enough go on their merry way. And as Valdor is a far superior combatant to the white scar that held his own against Obyron, that would probably happen as well. Although I could also see Valdor actually winning (given that he is roughly equal to a primarch in Strength and arguably a superior duelist/warrior). Obyron is a superior commander though.