When RK says:
"The use of two instruments of the same colour in octaves, e. g. 2 flutes, 2 clarinets or 2 bassoons etc., if not exactly to be avoided is certainly not to be recommended, as the instruments, playing in different registers will not correspond one with the other."
Does he mean e.g. 2 flutes playing an octave above e.g. 2 clarinets OR does he mean 2 flutes playing an octave apart doubled by 2 clarinets playing an octave apart OR is he simply referring to e.g. 2 flutes playing an octave apart?