I have read the white papers on how to optimize an application for SLI/CrossFire, and while X-Plane isn't quite a laundry of SLI/CrossFire sins, we're definitely an application that has the potential for optimization.
Now normally more hardware = faster framerate. In particular, the limiting factor of filling in a big display with high shader options and full screen anti-aliasing can be the time it takes to fill in pixels, and more shaders mean more pixels filled in at once.* Why doesn't having an entire second GPU to fill in pixels allow us to go twice as fast?
The answer is: coordination. Normally the process of drawing an X-Plane frame goes a little bit like this:
- Draw a little bit more of the cloud shadows into the cloud shadow texture. (This is a gradual process.)
- Draw the panel into the panel texture.
- Draw the world (as seen from below the water) into the water reflection texture.
- Draw the airplane shadow into the airplane shadow texture.
- Draw the entire world using the above four textures.
In fact, the total dynamic textures can be more so - if you use panel regions, there are 2 panel textures per region, and if you use volumetric fog, there are two more textures with interim renderings of the world, used to create fog effects.
Okay, so we have a lot of textures we drew. What does that have to do with multiple video cards?
Well, one reason why dynamic textures are normally fast is because, when a dynamic texture is finished, it lives exactly where we want it to live: on the video card!
But...what if there are two video cards? Uh oh. Now maybe one video card drew the water, and another drew the clouds. We have to copy those textures to every video card that will help draw the final frame.
There is a sequence to draw the right textures on the right card at the right time to make X-Plane run faster with two video cards...but the video drivers that manage SLI or CrossFire may have no way to know what that sequence is. The driver has to make some guesses, and if it puts the wrong textures in the wrong places, framerate may be slower, due to the overhead of shuffling textures around.
So SLI and CrossFire are not simple, no-brainer ways to get more framerate, the way having a faster GPU might be.
* If you have a huge number of objects, your framerate is suffering due to the CPU being overloaded, and this is all entirely moot!