heroes with same skills

Hey all, I was messing around with my party, adding a supremacy hero. and I noticed that dream dominator (mona evo) and shadow kitsune have the same skills, lowering top 2 attackers atk to 0 etc. Does this stack? so, my mona hits 1st and my kitsune hits 3rd, so will this reduce 4 attackers atk to 0 or does kitsune's skill just not happen? Also I have stella (starcaller evo) and I've got enough supremacy certs to get thanatos, since they both give supremacy, will they, not cancel out, but conflict with each other or because stella gives to 2 randoms and thanatos give to the 2 highest atk will there be no conflict?

Sorry to make your brain bleed so early, but I don't want to waste these supremacy certs if it's not going to work out for my party.

TIA