text: >Pokémon Emerald onwards, two Trainers who see the player at the same time will be engaged in a Tag Battle. If the player has only one conscious Pokémon, Trainer classes who will only engage the player in Double Battles will ignore the player and comment that the player should bring two Pokémon to battle them - Bulbapedia
  I was thinking of the older versions. Ruby and Sapphire. xD
  Well, they'll ignore the player. If you go up to them, they'll comment that you will need another Pokemon.
  Hope I helped. :)