I could be mistaken, but I believe SLI and/or Crossfire = Bad for overall hash rate. That said, if SLI can disabled, the fact that the motherboard doesn't support crossfire shouldn't be a problem if not attempting to use it anyway. However, I'm not 100% certain that all boards support multiple video cards anymore. They certainly all did prior to SLI and Crossfire, and the addition of those technologies shouldn't prevent one from using multiple cards independently, but that doesn't mean any given chipset or motherboard manufacturer couldn't screw it up. Regarding ASUS, watch your step with them. I bought a P5WD2 Platinum specifically because they advertised that it supported a SATA multiplier. I bought a multiplier with the matching chipset and it wouldn't work. Support was no help at all and basically ultimately told me that the motherboard didn't support multipliers and engineers were working on it. Because of this, I had to spend several extra hundred dollars to buy a RAID controller to connect everything I had purchased for the build and lucked out in managing to convince the company that sold me the multiplier to take it back.
Well I use currently Crosshair IV and 2 AX750 Powercolor.http://rog.asus.com/Product.aspx?Pid=40http://powercolor.com/us/products_features.asp?id=270#Specification
Of course with overclocks, it give me 170 mhs each so 340mhs.
Without crossfire GUIminer don't see both cards.
But I didn't tried under linux yet. (but other application including windows work well, and multi monitor support)
I tried solo card and it's the exact same performance.
Of course because both card are close there is more heat but you can resolve that with some great low price case (looking nice too) or with a mcguiver fan.
mcguiver fan : http://forum.bitcoin.org/index.php?topic=31383.0;topicseenSolutions for you
Do more testing, follow those "paranoiac" instructions carefully.
1. Remove any graphic cards, boot, wait 5 real minutes.
2. Reset the bios with the jumper. Situed south-east of the board, look in your manual for more details but it's pretty simple.
3. Install your card on the second slot ONLY, let the first one empty.
6. Profit !
By now it should work, if not, for an ultimate test, I suggest you to install Windows on a partition or another drive, as you want but make windows 7 running. Install drivers, see what's is going on.
I know linux can be problematic with drivers, for some reason you might have a compatibility problem or he just don't like to have both. Anyway if you can't go in your bios with the second card in the second slot, it's not an OS or driver problem. Try your new card in the first slot only, reset bios, if no result test your card with another board, computer (friends!!) If nothing work, RMA.