1. A method by a terminal in a wireless communication system, the method comprising:receiving, from a master base station, a message to add a cell of a secondary cell group (SCG) associated with a secondary base station for the terminal;
starting, in case that the message includes information for synchronous reconfiguration to a primary SCG cell, a timer with a timer value included in the information for the synchronous reconfiguration;
transmitting, to the master base station, a response message in response to the message; and
initiating, in case that the message includes the information for the synchronous reconfiguration to the primary SCG cell, a random access procedure on the primary SCG cell among SCG cells, based on preamble information included in the information for the synchronous reconfiguration;
wherein the timer is stopped in case that the random access procedure on the primary SCG cell is completed successfully.