In a wireless communication system, a user equipment, already configured with a first medium access control (MAC) entity for a master cell group (MCG) associated with a master base station, capable of configuring a secondary cell group (SCG) associated with a secondary base station based on a radio resource control (RRC) message received from the master base station. The user equipment can configure a second MAC entity for the newly configured SCG. Through a second RRC message from the master base station, the user equipment can then add a new SCG cell to the SCG, where the second MAC entity is commonly configured for one or more secondary SCG cells belonging to the SCG.