shouldn’t be difficult, in general once installed it should be available as audio device on system, so you choose it as the audio device for the os, then in the software you create as many outputs as you want, when choosing interface as an output it should list all available outputs so you can select the ones you want, after that you apply a headphone profile. I only have 2 outputs on my scarlet solo but I imagine if you have separate outputs on the rme they should be available…
checkout this video, it might help: