As per I know RACH happens after the SGNB addition is completed.
Secondary Node Addition Failure
The procedure to add a seconday node may fail. One cause of failure is an unsuccessful random access on the NR cell. Random access for SN addition is supervised by the timer GNBDUFuntcion.Rrc.t304, which is set in the gNodeB and signaled to the UE in the RRC Reconfiguration message. If t304 expires before the random access is successfully completed, then the UE reports a radio link failure (RLF) to the eNodeB.