X99 and the Haswell-E platform is launching soon an. The 8-core monster will have 40 PCI-Express lanes for plenty of multi-GPU usage, yet the cheapest 6-core processor SKU called Core i7 5820K would get a smaller number of PCI-Express lanes as was rumored earlier. This however never has been confirmed, until now that is.

Gigabyte posted X99 Motherboard Manuals on their website showing exactly that information. So the Core i7-5820K features 28-lane PCI-Express lanes. That means you'd have one x16 Gen 3.0 slot and then your second card at x8 Gen3. It's still a truckload of bandwith that will not effect performance TBH. Here's a snippet from the manual as spotted by a TPU user in their forums:

• 2 x PCI Express x16 slots, running at x16 (PCIE_1, PCIE_2)
  For optimum performance, if only one PCI Express graphics card is to be installed, be sure to install it in the PCIE_1 slot; if you are installing two PCI Express graphics cards, it is recommended that you install them in the PCIE_1 and PCIE_2 slots.
• 2 x PCI Express x16 slots, running at x8 (PCIE_3, PCIE_4)
  * The PCIE_4 slot shares bandwidth with the PCIE_1 slot. When the PCIE_4 slot is populated, the PCIE_1 slot will operate at up to x8 mode.
  * When an i7-5820K CPU is installed, the PCIE_2 slot operates at up to x8 mode and the PCIE_3 operates at up to x4 mode.

Gigabyte posts X99 Motherboard Manuals Leaks Info