Rule 465.2.c.7
If multiple Units have abilities or effects that require a player to assign them damage with the same priority, that player may assign damage to those units in any order. Example: A player is assigning damage to the following units: two units with Tank ("I must be assigned combat damage first.") and one unit with no abilities. That player chooses one of the units with Tank and assigns combat damage to it. Then they must assign any remaining damage first to the other unit with Tank, then to the unit with no abilities.