I don't see why not, anymore, based on Blizzard's changing philosophies.
Anyone else remember when they expressly told us that PvE->PvP transfers would never be possible back in Vanilla? Then we had faction changes, and sharing capital cities. Now we even have a cinematic (albeit a fairly harmless, non-serious one) where a Human and an Orc are drinking together in a tavern.
Is it such a stretch to think that a Horde and Alliance character would utterly refuse to join together even for half an hour to face a common foe? We hardly even fight anymore, it's not as if every single character in the world has to be an unreasonable, racist asshole, right?