It was necessary for Mr. Lorry to wait for Lucie in Dover to travel to Paris because Mr. Lorry had figured out a plan to rescue Lorrie's father who has been imprisoned in La Bastille. Lorrie's father's name is Dr. Mannette, and he was a victim of the power of absolutism and, along with Lorry and Lucie, depended on other secret groups whose goal was to overthrow the monarchy. The monarchy used the Bastille as its center for punishment and imprisonment. Sometimes the punishments were fair, but somethings the punishments were cruel, and vindictive.
Dr. Mannette had not met his daughter Lucie yet, and this would be the first time they would meet. Another thing we know about Dr. Mannette is that his punishment was wrongful, that he used to be a very important member of society, and that after all his years in prison he has lost his mind.
Therefore, it was necessary to have more than two people in the search party to join the plans to rescue the doctor.