A method for a user equipment (UE) operating in a wireless communication system including a first base station (BS) and a second BS. The UE includes two medium access control (MAC) entities: one MAC entity for the first BS and one MAC entity for the second BS. The method includes mapping a first logical channel group (LCG) including one or more logical channels for transmitting first data to the first BS to the one MAC entity for the first BS and a second LCG including one or more logical channels for transmitting second data to the second BS to the one MAC entity for the second BS; based on the first data belonging to a logical channel in the first LCG having a higher priority than priorities of other logical channels in the first LCG for which data is already available for transmission, triggering, by the one MAC entity for the first BS, a first buffer status report to the first BS independently of logical channels of the second LCG; and based on the second data belonging to a logical channel in the second LCG having a higher priority than priorities of other logical channels in the second LCG for which data is already available for transmission, triggering, by the one MAC entity for the second BS, a second buffer status report to the second BS independently of logical channels of the first LCG.