cr:7.0.5e

If exactly one card is put onto a chain link as a defending card it is considered to defend alone. If two or more cards are put onto a chain link as defending cards in the same event, they are considered to defend together.

Example: If a player declares two cards during the defend step of combat, those cards are considered to defend together. Then, if the player plays and resolves a defense reaction, that card is considered to defend alone because it is just one card being added as a defending card - despite there already being defending cards on the chain link.