Driver is only beta and is most likely the problem.
I did not look at your computer specs for PCIe speeds.
To OP, PCIe 8x did not used to be a problem, however, these GPUs push so much communication through PCIe that they may take a larger hit than older video cards.
I am familiar with older dual video card solutions, but this is too new for me. One PCIe v2.0 16x and one 8x used to drop performance around 8% on average. Two PCIe v2.0 8x, I am not sure, but the faster the card the greater the loss, but it has to be a FAST card to lose more.
The R9 290 or R9 290X should be hampered more than 8% on PCIe 8x especially if it is PCIe version 1.
I missed you saying you have PCIe v2.0, sorry. As mentioned though, these cards push a LOT of data and I expect they are the first generation that can saturate PCIe 2.0. That said, you will lose. Not sure how much, last generation fast cards used to lose, on average, 8% with one 16x and one 8x. Slightly more with two 8x, but not much more. The R9 290 and 290X push a great deal of data through PCIe, and I am not sure how much you will lose at this time. You might try a google search, unless someone posts here with hard numbers.
As far as how much data throughput will be slowed, it also depends upon the entire motherboard/CPU as well as PCIe v2.0. It looks as though your computer is borderline for PCIe data throughput. Keep what you have and let us know what percent difference you have with 1 card and 2. That information will help everyone with the same question you have.
Edited by Dan848 - 11/9/13 at 10:22am