I've come to find out that if I want to have the Discord overlay functional in Overwatch, I need to launch the programs in the order Discord > Overwatch > Action! (Discord has to be running already when the game is launched for the overlay to function, that's normal for that program)
If Action! is running already when the game is launched, the Discord overlay will not appear, nor can it be invoked, no matter what settings I mess with. But if I execute them in the order above, both programs' overlays function perfectly.
Is there anything that can be done so that I don't have to deliberately close Action! and reopen it again when I want to have Discord overlay functionality?