To my understanding that means only ONE person in the conversation has to know it is being recorded.968.31(1)
(1) Except as otherwise specifically provided in ss. 196.63 or 968.28 to 968.30, whoever commits any of the acts enumerated in this section is guilty of a Class H felony:
(2) It is not unlawful under ss. 968.28 to 968.37:
(c) For a person not acting under color of law to intercept a wire, electronic or oral communication where the person is a party to the communication or where one of the parties to the communication has given prior consent to the interception unless the communication is intercepted for the purpose of committing any criminal or tortious act in violation of the constitution or laws of the United States or of any state or for the purpose of committing any other injurious act.
The site listed seems to support this conclusion also.