Order and Destruction only find common cause in the most desperate of circumstances. Units from the Forces of Order always treat units from the Forces of Destruction as being desperate allies, and vice versa.
Desperate Allies work exactly like, except that they:
Have to take Dangerous Terrain tests when fleeing through allies.
Are counted as 'enemy' units from the point of view of targeting spells or special rules.
Are treated as Expendable by their allies.