My thought is as follows: The Mathmatical Necron mind would probably Prioritize the Tyranid Bioship as the greater immediate threat to "their" world. (The Human cockroaches could be anhihilated or enslaved later. The Tyranids, Being highly psychically attuned would be anathema to the the necros!) This would make The Necrons an odd "ally of convenience" as they engaged the Bioship. The trick would be finding the right time to turn on the Tombship before it decides to eat their star!
All we need now are some Blood Angels to show up and broker the initial deals...and bring Termies to bust up the Cairn when convenient.
At least one company of BA and a Battlebarge! With any luck there will be that much less red cheese in the galaxy! (Is my Distaste for the Blood Angels showing? Have trouble accepting twilight vampire space marines...Just sayin!