In this picture you see the problem:

in both "edge"barrels there are terminus and quicksort modules, the middle chest is full, now both chest keep sending the items to each other all the time.

any way to avoid/fix that behaviour?

Err, that's how quicksorts work. They send to anything that's > default route priority.
So 2 chests/barrels/whatever each with a quicksort and a itemsink or terminus will happily send items to each other all day long.

hmm okay, so there's no way to avoid that - hmm perhaps I should simply use gates/wires to disable the pipes as long the middle chest is full

yes thats a good idea with the inifinite supply mode, but in my current setup I would need an "active supplier module". so I simply disable the quicksort modules via bc-wire/gate currently, thats working okay for me

@Flow86 suppliers now have an infinite supply mode, so provide the edges and infinite supply the middle :)