Whether or not a person can have two residential phone lines depends on the telephone company. For example, Time Warner allows customers to have two residential lines.
Some customers want an extra residential phone line in order to keep one free, and use one for Internet service. Other people may want an extra phone line if they have multiple people in the home that use the phone often. When a person wants an extra phone line, the jacks may need to be wired in order to accommodate this. The phone company also will charge extra for each additional phone line in the home.