The PC launch of Arkham Knight has been troubled since the start, and that's quite an understatement.
Long story short, the PC version of the game was ridden with bugs that ruined the experience for a lot of gamers. The game was pulled and then relaunched but problems persisted. Earlier this month, Warner Bros. announced it will not be able to fix all problems with the game, and offered buyers a refund until late December.
The game studio is still working to further polish the game though, some progress may be made here and there but another blow was delivered as one of the developers just gave an update about the multi-GPU situation on the Steam Community boards. Apparently, work on multi-GPU support has ceased because it was not practical:
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.