If the situation should arise, that both an Ork-Fleet and an Imperial Fleet meet while fighting against a big Tyranid-Fleet. Could it be possible for both to leave with a symbolic nod and not kill each other afterwards?
Or picture the Kill-Team, engaged in a fight with a Tyranid-prime on a hive-ship, when suddenly an Ork-team bursts into the scene and helps the Marines, since both want the threat of the Nids being taken care off. Could they afterwards both leave without killing each other?
I ponder on using such a situation in Oblivion's Edge to put a bit roleplaying into it and to provoke discussions between the different chapters and maybe a coverup story afterwards to not let this event come to the ears of he inquisitor.
(Or maybe even resulting in a follow-up adventure against the Ordo Hereticus or so...)