If the motherboard is SLI-ready, it will come with an SLI connector and you'll have no problem with the 2nd 680 whatsoever. SLI requires x8 speed at a minimum. You will see no performance hit. With your board, when the 2nd PCIe 3.0 x16 slot is populated, both PCIe 3.0 x16 slots run in x8 mode (x8/x8). From the Gigabyte description of your motherboard at http://www.gigabyte.com/products/product-page.aspx?pid=4139#sp:
1) 1 x PCI Express x16 slot, running at x16 (PCIEX16)
* For optimum performance, if only one PCI Express graphics card is to be installed, be sure to install it in the PCIEX16 slot.
2) 1 x PCI Express x16 slot, running at x8 (PCIEX8)
* The PCIEX8 slot shares bandwidth with the PCIEX16 slot. When the PCIEX8 slot is populated, the PCIEX16 slot will operate at up to x8 mode
In other words, both slots run at x8 when you pop the second 680 in. This will work fine.