Errantry

• WWizards of the Coast2008-10-01
  Official Ruling
  “Can only attack alone” means the enchanted creature can’t be declared as an attacker during the declare attackers step unless no other creatures are declared as attackers at that time (either by you or your Two-Headed Giant teammate).
• WWizards of the Coast2008-10-01
  Official Ruling
  If the enchanted creature attacks alone, then a spell or ability causes another creature to be put onto the battlefield attacking, that’s fine.

Rulings via Scryfall