Can you summon another unit directly into a battlefield where you just initiated a showdown, or must you wait until after the showdown is finished and you've conquered that battlefield?

Ruling: You cannot use standard moves to move units to a battlefield once a showdown has started. You must move multiple units at once before the showdown begins if you want them both there.

Nuances:

• You can still move units to a battlefield during a showdown using spells that move units, just not standard moves
• You can play units to a battlefield during a showdown if you already control that battlefield (usually when defending) AND the unit has action/reaction itself (like Shen) or you play it via another action/reaction (like Stacked Deck revealing Nocture)
• A showdown happens whenever a unit is moved to a battlefield the unit's owner doesn't control - this can happen multiple times during a turn
• Showdowns can also happen in the end phase (e.g., if you draw Deadbloom from Aurora and play it to an enemy battlefield)