WAV, if a bass and tenor sang the same tune at different pitches, they'd have to be an octave apart to sound right. This may be classified as harmony of a very elementary type, but most people would regard this as unison. It's the same as when you get (say) men and women singing "Happy Birthday" together - they sing in different octaves but it hardly rates as harmony, sing they'll both be singing a G or whatever at the same time. To most people, harmony means a different interval from the octave.

Marje