Oracle does not support simultaneous update of 2 tables, it would be necessary to either perform 2 transactions or create a precedent that performs this procedure for you:
UPDATE SAC_RV_DIVIDENDO SET DT_EX = '20190110' WHERE ID = 53136;
UPDATE SAC_RV_DIVIDENDO_QTD_POS SET DT_EX = '20190110' WHERE ID = 53136;
COMMIT;
Or
CREATE PROCEDURE UPDATE_SAC_RV_DIVIDENDO(P_DT_EX IN VARCHAR2,
P_ID IN NUMBER) IS
BEGIN
UPDATE SAC_RV_DIVIDENDO SET DT_EX = P_DT_EX WHERE ID = P_ID;
UPDATE SAC_RV_DIVIDENDO_QTD_POS SET DT_EX = P_DT_EX WHERE ID = P_ID;
COMMIT;
END;