The Intel 1155 chipset (which the z77 motherboards are based on) only provides 16 PCIe lanes. Any decent mid-range motherboard will allow these lanes to be split between two PCIe slots, allowing each card to have x8 lanes.
Some top end motherboards (like the one you quoted) add a PLX chip which basically allows both cards to run at x16, BUT all it does is give the 16 lanes (which it's limited to by the processor) to whichever card needs it at the time. In otherwords, you don't have 32 lanes, you just have the full 16 given where they're needed. NOTE: the PLX chip does introduce a little latency which actually (very slightly) reduces your performance.
The good news is that there are basically no real word situations where even top end cards require the full PCIe 3.0 x16 lanes. PCIe3.0 x8 is more than enough for any video card.
Even if you get 2x GTX Titans and OC em, they'll run just fine on PCIe 3.0 x8 lanes.