Multi-stream transport over Thunderbolt. Basicallly split the screen in half, so each stream is under the maximum bandwidth.
Each stream = 1 display.
But I'm having trouble believing this thing can do TWO 5k displays simultaneously at 60Hz. According to the review on Ars Technica, the first Thunderbolt controller has four lanes of PCIe, while the second Thunderbolt controller only has 2 lanes of PCIE 3.0 = 16Gbps max bandwidth.
4k @ 60Hz requires 14Gbps, and 5k at 60Hz is 80% more pixels, so around 25Gbps. That will be closer to 30Hz on the slower controller, unless you know something I don't?