It may be due to PCIe lanes. The platform has 40 Total.
I don't know yet what the GPU's use but they could be using 16 lanes each or 8 lanes each. If they use 16 each then that's 32 out of the 40 gone leaving only 8 for Thunderbolt, USB 3.0 and the PCIe SSD.
Actually, it has 48 lanes.
40 lanes of PCIe 3.0 from the CPU package, and 8 lanes of PCIe 2.0 from the C602 chipset.