Ok, the game release pretty much is a disaster eh ? After all that happened, now you do not get multi-GPU support either. This actually is an Nvidia Gameworks supported title ? The promised re-release of Batman: Arkham Knight on Steam did not pack any performance improvements at all (compared to its Interim patch). Now SLI and Crossfire is not supported either.

Today Warner Bros stated that there won't be any patch to add multi-GPU support :-(

We’ve been working with our development and graphics driver partners over the last few months to investigate utilizing multi-GPU support within Batman: Arkham Knight. The result was that even the best case estimates for performance improvements turned out to be relatively small given the high risk of creating new issues for all players. As a result we’ve had to make the difficult decision to stop work on further multi-GPU support. We are disappointed that this was not practical and apologize to those who have been waiting for this feature.

Not even Multi-GPU Support For Batman: Arkham Knight