It depends on the proportions you use I would have to think. It's like adding liquor (rum) and soda (coke). The overall drink is still carbonated even though the rum is not. Also the temperature of the two liquids would come into play as the colder they are the better CO2 will stay in solution.
Since apple wine is a little higher in alcohol than typical draught ciders, I'd say just cut back on the apple wine and add more stout. That would compensate for the apple wine not being carbonated. Sounds tasty to me.
Cider is a substitute for champagne in this cocktail, and when this is done it is called a "Poor Man's Black Velvet". Apfelwein should be fine. Just make sure you pour the heavier / higher FG beverage first, so that the lighter / lower FG beverage can float on top.
I didn't see any trick question. Poor Man B.V.'s are tasty. To not mix them is like saying..."Don't add ice cream to apple pie because you don't want to waste good apple pie." ???