One other thing to think about. A queen excluder is designed to keep a queen in a box, but if there is another queen on the other side, I doubt that it is thick enough to keep 2 queens apart if they want to fight. Yes, they may not be able to cross it, but I bet that they can get enough of their abdomens through to sting their opponent. Most of the stacked nucs that I have seen have a spacer between them, just to prevent the queens from reaching across the gap.
Be careful! Losing a queen is $20-40. Losing a nuc is $150+. Is it really worth it? 