[Suicide Junkie]

If you trust the "to-hit bonus" calculation switch in the ship design menu, they do stack.

It says my LC has +10 defence. When I add ECM III, it says +70. When I add stealth armor it says +85. When I then add scattering armor, it says +100.

I believe that it does stack, because I went up against a max-tech Phong DN in a crap-tech LC, it was hitting all the time. When I added both armors, they finally started to miss about a third of their shots.