Your observation is not a bug, as AMM isn’t designed for this, nor would it be particularly well-suited for it.
(Couldn’t you try it by simply plugging your cables from 13/14 into the combo jacks of two with AMM usable inputs?)
What you’re missing is a ducking function, and there are already several feature suggestions for this, such as the one at the link.
However, A&H has already admitted that they (still) don’t know if it would even be technically possible to implement.
And when they write something like that, it’s usually not a good sign pointing towards “yes”.