Loads of boards have one true 16x (electrical and physical) PCIE slot. The second slot is typically 8x electrical, 16x physical and then when you plug in the 2nd card into the slot, you end up with 8x and 8x -- PCIE 3.0.
What I described is very common on Z77 boards, so don't worry about it.
Well you can get a motherboard with a PLX PEX8747 chip on it. Though truth be told they only multiplex the existing lanes into additional lanes. They feed through the same x16 pipe into the CPU. They only really allow for dynamic lane switching and even lane distribution amongst more than two PCIe devices.