In a 2v2 game, can a player summon units to a battlefield that their ally has conquered and already has a unit on?

Ruling: No, you cannot have units on a battlefield that your ally controls.

Nuances:

• This also means you cannot use Sprite Summon to an ally's battlefield
• Seals can only add Power, not Energy
• Power symbols (fury, calm, body, mind, chaos, order, universal power) always mean Power
• Energy symbols (0, 1, 2, 3, etc.) always mean Energy