If memory serves me right it's a little bit different ...
All channels are instantly being transferred, so it should be sufficient to run audio through one stereo pair and then to watch out in the driver settings for CRC errors and to listen whether you hear clicks and pops.
That's also why RME points out that the RME Recording interface has (in all those cases) more channels / data to transfer,
when users note, that it worked flawlessly on a 2 or 4-channel XYZrite. Some computers can't cope with permanently having to transport more audio bandwidth in time. But if you have a good computer, then RME is able to transport 68/68 (in/out) through USB2, see MADIface Pro !!
The only driver which optimizes the amount of channels being transferred over PCIe is the HDSPe MADI FX.
And with the Firewire driver it was possible (not sure whether for all FW interfaces) to select, whether to use / transfer only analog or all type of channels (incl digital channels like e.g. ADAT) through Firewire.
BR Ramses - UFX III, 12Mic, XTC, ADI-2 Pro FS R BE, RayDAT, X10SRi-F, E5-1680v4, Win10Pro22H2, Cub13