There is only a single full bandwidth PCIe slot, so no, you can not xFire or SLI it.
One is PCIe 3 and one is PCIe 2 because that is what the engineers wanted to make lol.
Coming from the CPU there are only a total of 24 PCIe lanes, that have to be distributed between Graphics, Peripherals, Storage. Those have to be divided in whatever makes sense for the purpose of the MB. Small form factor MB's rarely have SLI / xFire configs due to the purpose of being small.