Well, google is your friend.
EVGA forums, scroll down to 9th reply by user HeavyHemi:
http://www.evga.com/forums/tm.aspx?m=1421266&mpage=1
Also note that the prevailing advice in that thread if you want to really crank up the resolution while also keeping the detail settings high, is to get a single card with the highest amount of RAM you can afford. So it's not only modern flight sims that work this way (RoF also had a lot of problems with SLI early on, but i can't comment on its current state because i don't have it on my PC), it seems to be a more widespread trend in other games too.
Tom's Hardware SLI and Crossfire FAQs:
http://www.tomshardware.co.uk/forum/...crossfire-faqs
Incidentally, in the above link you can also find this little gem:
So FSX, a flight sim that was very demanding in graphics and CPU until hardware could could catch up with it, doesn't benefit from it. Sounds very familiar. Also note that Starcraft2 is a blockbuster AAA title.
Both of them are made by companies that could throw tons of cash on the issue. FSX is getting old and microsoft is more concerned with selling DLCs for its new MS Flight, but this wasn't always the case. Yet, they didn't fix it.
Also, SC2 is at its peak and its only part one of a trilogy, with a highly competitive multiplayer scene (think professional gamers who get paid like footbal players to take part in tournaments, etc) and the company behind it (Blizzard) has the enormous world of warcraft MMO cash-cow at its disposal and raving mad fans who buy everything they release (eg, the recent Diablo III).
If these guys can't do it or won't spend the time and money to, then the only reason i can think of is that SLI/Xfire setups are a bit too particular in terms of how you code your game in order to work correctly. It seems like the game has to be written around it and since it's a somewhat rigid and not so evolving technology (the cards evolve, but the technologies that pair them not so much), maybe it's not worth the compromises in other parts of the engine?
I'm just thinking out loud here, but the whole thing seems to completely debunk the entire "two cards = double the performance" logic. I've been ordering my PC components separately since forever and the only people i routinely see going for SLI setups are those that primarily focus on action/shooter games (simpler engines, small maps, elementary game mechanics, so all the PC has to do really is to run good graphics at a high frame rate).
The bottom line is, just because we might have some extra money to burn on a PC build doesn't mean we should go for the most expensive options. They might be kind of specialised in what they work well with.