If you have a look at this link:http://dlcdnet.asus.com/pub/ASUS/mb/socketAM3/M4N75TD/ie5353_m4n75td_insert.pdf
It says that the slots work in X8 mode regardless of single or dual card configuration. Weird, I have never seen a motherboard like this before. It has 16 lanes, but it seems that 8 of them are hardwired to one slot and 8 to the other, it can't allocate all 16 lanes to one slot.
It says much the same thing in the manual I downloaded from ASUS.
I wouldn't worry about it too much as HardOCP just did an article showing that SLI and CrossFire are not handicapped by running X8/X8 versus X16/X16. If running X8 on two cards doesn't bottleneck performance, X8 on a single card shouldn't be an issue.
Heres that article:http://hardocp.com/article/2010/08/16/sli_cfx_pcie_bandwidth_perf_x16x16_vs_x16x8