Today we see many forums with comments that dual and triple video cards are not being used. Support for a 2-way, 3-way or even 4-way is entirely by design of the game engine.
The game engine developer usually has only one card installed. Very few developers have a machine with GTX 690 SLI cards installed. The Kepler cards are designed for up to DX11 shader model 5.
Things seem to be bad enough that dual cards should be avoided in favor of one extreme card. This is the mainstream graphics card market where over 99% of machines use one video card.
Back when we had the GTX 260 SLI setup, it made more sense considering the capabilities of the cards to handle 1920×1080 rendering.
The GTX 1080 Ti can exceed GTX 770 SLI while using less than 50% of the power. The Vega RX64 cards are so powerful they can handle Battlefield 1 at 3820×2160 with one card.
The old alternate frame rendering has become unneeded as modern cards are so powerful. The Unreal 4 engine does not support multiple cards;. The Frostbite 3 engine does support dual cards.
DX12 has multiple explicit GPU capabilities but not many games have that capability as of yet. DX12 can take advantage of cards with different designs and performance. So far only one game: Ashes of the Singularity has support for using 2 disparate video cards.