You want a board that has PCI-e slots that do 16x/0x and 8x/8x. That means when one GPU is in, it will get 16 pci-e lanes and the other (empty) socket will get zero. If both sockets are full, each will get 8 pci-e lanes.
Some cheaper boards limit their 2nd slot to 4x which generally isn't enough for SLI/Crossfire.
Something like this
would work. Although there are cheaper options.