A method of handling simultaneous communications in a communication system comprises a user equipment (UE), a first base station and the second base station, where the UE has been connected to the first base station. The method comprises the first base station determining to allocate a cell of the second base station to the UE; the first base station transmitting a cell addition request message to the second base station in response to the determination, for requesting the second base station to allocate radio resources of the cell of the second base station to communicate with UE; and the first base station transmitting a first radio resource control (RRC) message to the UE, wherein the first RRC message comprises information related to the security key and information related to the security algorithm.