There are generally no routing possibilities at all unless there’s some sort of DSP mixer on the interface. You just get multiple “independent” (synchronized) devices, or a device with more than two channels, depending on driver and API.
From a technical point of view, routing has the same issues as proper mixing, and can’t be done without explicit hardware support, as doing it in the driver would add latency, and/or increase the risk of glitches.
Just routing one stream to multiple channels should be safe, though (just have the driver write the data to multiple buffers), but I haven’t seen anything like that supported outside of ALSA/Linux. Definitely possible to do on any OS, but that would be entirely driver dependent.
The safest bet without hardware support would be to do it in the DAW (provided the audio comes from that one DAW), since that shouldn’t add latency or anything. You just need the ability to route the audio to multiple channels/busses/whatever, that you can then assign to the desired interface outputs. Definitely possible in Pro Tools and Cubase, and I would expect most other DAWs to be able to do it as well, one way or another.