Atlas: “no! Put the red portal there!”

P-Body: “no you dummy! I can’t cause if I walk through the emancipation grill, it’ll deactivate!

the two argue for a while before GLADOS speaks up

GLADOS: “since you two dumb robots can’t make up your mind, you’re getting a third partner…by the way, you two are insufferable…”

then a tube drops in the room, and you fall out alongside your own portal gun