Your motherboard has two pcie 3.0 and at dual mode 8x 8x, no bottlenecks for dual gpu setups, but i don't recommend adding another 460 gtx, get a better card from current generation of nvidia cards, a decent upgrade will be the 660 gtx/660 ti/670 gtx.
Keep in mind if you want to add another one later for those mentioned, you need a psu with 600-750w depending on which card.
Cool! Just to see if I understand correctly, if I were to buy a 660 gtx, could I run it at dual mode with my current 460 gtx? This intrigues me greatly