Nico as primary commander with Kinnara secondary makes a lot of sense since Nico also has a huge target debuff in his active skill. So, either Kinnara or Nico will use their skill and debuff so the target legion takes a lot more damage and then the other one hits with the active skill

Rattle Spear is the kinnara artifact and as it synergizes with DEF Break that Kinnara can apply