1. A method for a user equipment (UE) communicating with a first base station (BS) and a second BS simultaneously in a wireless communication system, the method comprising:configuring, based on a radio resource control (RRC) signal from the first BS or the second BS, that uplink (UL) data is to be transmitted to the first BS for when an amount of data available for transmission in a packet data convergence protocol (PDCP) entity is less than a threshold;
receiving a first PDCP data from an upper layer;
transmitting a buffer status report (BSR) for the first PDCP data to request an UL grant to the second BS;
receiving a second PDCP data from the upper layer;
receiving the UL grant from the second BS in response to the BSR; and
transmitting both of the first PDCP data and the second PDCP data on the UL grant to the second BS even when an amount of the data available for transmission in the PDCP entity at the time of a transmission of the first PDCP data and the second PDCP data is less than the threshold.