def get_first_second_usernames(user_name_list):
    """
    prompt for two user names sperated by comma
    check if input is valid and exists in list of user names
        if its not reprompt
    :param user_name_list - list of usernames in twitter data:
    :return first username, second username - strings of both users:
    """

    not_correct = True
    first_username = ''
    second_username = ''

    while not_correct:

        user_str = input("Input two user names from the list, comma separated: ")

        if ',' in user_str:

            try:
                user_str = user_str.split(',')
                first_username = user_str[0].strip()
                second_username = user_str[1].strip()
            except TypeError:
                print("Error in user names.  Please try again")

            if first_username not in user_name_list or second_username not in user_name_list:
                # your check to for correct user names goes here
                print("Error in user names.  Please try again")
            else:
                break
        else:
            print("Error in user names.  Please try again")


    return first_username, second_username