I think x8 bus lanes is the minimum. As these GPUs are CrossFireing via the PCIe bus, you want x16 on both for best performance. Your 2nd PCIe bus is x8, so both should run when in CrossFire at x8.
It would be worth taking out the 2nd GPU, and confirming it is seated correctly into the socket. Also might be worth checking in your ASUS BIOS and looking at what xlanes that is reporting too.
If your BIOS is reporting the same, this would suggest the card is damaged, or the mobo is. Or just that not all of the x8 contacts are touching the card as they should so the mobo thinks you only have an x2 card.
Can you confirm the same is being reported when you run the GPUz render test? (?) should have an option to test CrossFire.
As far as I know, AMD has no PCI-E lane requirement, unlike Nvidia, but for getting the most of your 290x pair, x8 on PCI-E 3 would be the lowest you would want to use.
Regarding the reported lanes via GPU-Z, as tomtalk24 suggested, you should check what is shown when using the "PCI-Express Render Test".
Another option would be disabling the ULPS: