Combo Attack — ruling 1

You can’t cast Combo Attack without targeting two creatures your team controls. If one of those creatures is an illegal target as Combo Attack resolves, the other will still deal damage equal to its power.

Combo Attack : règles, fonctionnement et interactions (MTG)

Combo Attack is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Two target creatures your team controls each deal damage equal to their power to target creature.

How it works : Combo Attack

Original English text — Wizards of the Coast / Scryfall

#1June 8, 2018· Wizards of the Coast
You can’t cast Combo Attack without targeting two creatures your team controls. If one of those creatures is an illegal target as Combo Attack resolves, the other will still deal damage equal to its power.
No other card is named in this ruling.