Okay. Let's stop arguing about the stupid 40% blahblah technicality issues and go on to some more realistic issues.
Out of the six pairs, the only pair that actually gives a decent deck, especially since this is a duo deck tourney, is
Discord-Black Hole.
I suggest banning discord for a tourney like this. Duo decks are really fragile against discord, and along with Black Hole, this deck can counter most other decks quite easily.
If not banned, the tourney is basically going to be
-
competition where whoever brings out the discord earlier wins.
But then again, there are ways to counter discord.
1. Nova
2. Immolation
EDIT : Rather than banning a specific card, there are other ways. How about setting so that a player cannot use the same deck more than once in a round? (Rasta's suggestion)