Leadership is the only Sphere that can make a tournament-legal single-sphere deck with 2 Core sets.
Here's the breakdown with 2 Core sets (totals include Hero cards):
- Leadership: 48. (Needs 2 Gandalf cards to be legal)
- Tactics: 44
- Spirit / Lore: 45
Cards that need a third set for completion:
Leadership:
- Celebrian's Stone (U)
- Brok Ironfist (U)
- Grim Resolve
Tactics:
- Beorn (U)
- Swift Strike
- Stand Together
- Horn of Gondor (U)
Spirit:
- Dwarven Tomb
- Fortune or Fate
- Unexpected Courage
Lore:
- Henamarth Riversong
- Beorn's Hospitality
- Dark Knowledge
So the only solution for a legal deck outside of Leadership is a double-sphere deck.
On another note, at 4 players, making double sphere decks gets real interesting because you can still only have double sphere decks if the 4th player plays a triple or 2 players play mirrored doubles.
Luckily, it seems the first adventure pack will be out not too far from now.
Just an FYI