Are since there are multiple people Use is for one person For example Is Mrs Li in a big shop? Are Mrs Li and Kitty in a big shop?

Mrs. Li and Kitty ARE in a big shop. "ARE" indicates they are both in the shop. So the right one is: "Are Mrs. Li and Kitty in a big shop?"

To refer your second question, you are describing for two people "Mrs. Li and Kitty". In this condition, it is using in "plural" form, so the correct answer must be: "Are Mrs. Li and Kitty in a big shop? However, if you wrote with the verb "is", we say that it is a singular form. Then you can ask as below: Is Mrs. Li in a big shop? Is Kitty in a big shop? Is Mrs. Li or Kitty in a big shop?