I've long felt that multi-GPU systems are playing with expensive fire, and even worse can, like audiophilia, lead to an anxious, ongoing preoccupation with whether you have the 'best' at the expense of actually enjoying what you're playing/hearing. Even so, I sympathise with people who invested both money and time into their SLI or Crossfire setups, only to now be told that one of the biggest games of the year will effectively never support 'em. Warners/Rocksteady have officially thrown in the towel for Batman: Arkham Knight [official site]'s errant multi-GPU performance.
In response to the ongoing grievances over on the Steam forums, Warner Bros. representative 'wb.elder.pliny' dropped the bad news. Despite apparently working with hardware and driver manufacturers on the issue for several months, "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."
This follows on from the company all but abandoning the troubled Batman sequel to its messy fate even after a re-release after pulling it from sale and spending months trying to fix it, not entirely successfully. They did, however, claim that they'd still work on multi-GPU support; now that's been dropped I'd be surprised if much else happened to the game.
A sorry saga. And as much as this is a very special case, it only cements my opinion that it's best to stick to the best single card you can comfortably afford. I'll take slightly reduced settings over technical headaches any day of the week.
If you bought Arkham Knight from Steam at any point during its sorry release saga, unconditional refunds are being offered until the end of the year.