This is a landmark work. It was originally designed to played in the Basilica San Marco where there are two high galleries facing each other. The two ‘choirs’ of instruments (originally brass) would play with and against each other in imitation both melodically and dynamically. It must have been hugely impressive.
So, rather than being an octet as such, it would be more accurate to call it a double viola quartet. Magnificent! 5 mins. E/M