DragonFox69 said:
As @Kemonophonic: says the mixing side of it is called panning. The musical side of it depends on the notes, if they are both the same note but at different registers (ie: I and VIII) it's just called "playing octaves" but if the notes are different (ie: I and iii) it's called "Harmonising".

Yeah, that's it. Harmonising was the word I was looking for. I thought there was something more to it than just the panning.