Motherboards that support two 16x pci-e cards at full speed, and also actually manage to get the promised high throughput for cpu<->gpu transfers are quite rare, but if you manage to find one, you will get comparable speeds.
Wouldn’t you only need two 8x pci-e slots at full speed on the MB since the D870 only has a single 16x pci-e interface for both cards? Then wouldn’t having two C870s potentially have higher bandwidth then the D870?
But the D870 is PCIe v2 with double the bandwidth. Assuming your memory/host motherboard can feed it, the effective bandwidth should be the same as PCIe v1 x16 slot for each card in the D870.