In my experience, by the time you are ready to get the 2nd card, there's more attractive, affordable and more power efficient single cards available that will be the more attractive route to upgrade over trying to find a second matching video card to c/f.
I have 3x 6950s w/dual BIOS flashed to 6970's but Id be better off with a single 7970. When you look into my case and the "WOW" factor hits you that the case is full with beautiful parts then the reality comes into your mind that its not the most efficient nor is it the most powerful setup you could have for that amount of money now.
I've also lately had an issue with my system crashing and could be due to the multi card setup and the drivers. I just updated to the latest driver and it could be something else besides that but what I'm trying to say is, do you want to put your system together and use it with less problems or do you want to tweak and iron out issues along the way.
If you answered to put your system together and use it, go with a single more powerful card.