Can a player move a second unit to a battlefield to continue a showdown after their first attacking unit loses, and does damage carry over between combats?

Ruling: No, a player cannot move additional units into an ongoing showdown after the initial attacker is declared. After each combat resolves, surviving units heal all damage, so a second unit moving to that battlefield would start a completely new showdown against a fully healed defender.

Sequence:

• Attacker declares which unit(s) are moving to start the showdown
• Combat resolves with actions and reactions
• If units survive, they heal all damage during cleanup
• Only during a neutral open state can the active player perform another standard move to start a new showdown

Nuances:

• You cannot "dogpile" additional units into a showdown after it has started, even if reactions or action cards change the outcome
• Moving a second unit after the first combat ends is legal, but it starts an entirely new showdown against a fully healed defender
• If a showdown results in a tie and both units are destroyed, leaving the battlefield open, another unit can move in to conquer the now-open battlefield