Steam Community forum post from a WBIE developer account has the news that they are abandoning their efforts at adding working support for multiple GPUs in Batman: Arkham Knight (thanks DSOGaming). The game suffered from performance issues upon release, and while some of them have been addressed, word is SLI/Crossfire support will not be added:

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.