You want QSFP56-DD to 2x QSFP56 cables. The -DD is double density and it is a deeper physical connector used on the CRS804/ CRS812 (and lots of other switches.) Here is Rohit’s post on that if you want to learn more: QSFP Versus QSFP-DD Here Are the Key Differences - ServeTheHome - The short answer is that you are just splitting out the double density.
You also have to manually set each port to 200Gbps. The actual PCIe Gen5 x4 throughput you get is closer to 109Gbps IIRC but I might be a bit off there so you lose a small bit of bandwidth by doing both of the PCIe Gen5 x4 links into a single connector. For the GB10, it is well worth it to get higher density on the switch.
Just as a FYI - we have eight of them on a CRS804 and it works no problem. If you want, you can even put storage on the 10Gbps ports and feed storage in that way.
We are doing our video on the 8-node cluster today, so it should be out in two weeks or so (albeit likely when I am filming in Taiwan.)