Rule 802.3 · Attack Multiple Players Option
As the attacking player declares each attacking creature, they choose a defending player, a planeswalker controlled by a defending player, or a battle protected by a defending player for it to attack. See rule 508, “Declare Attackers Step.”
Subrules
Restrictions and requirements that don’t apply to attacking a specific player are evaluated based on the entire group of attacking creatures. Restrictions and requirements that apply to attacking a specific player apply only to creatures attacking that player. The entire group of attacking creatures must still be legal. See rule 508.1.