SELECT * FROM (
    (SELECT iurd1.dbo.Analogicos_2SS.QA_2SB_03_SENSOR_CO_1 as QA_2SB_03_SENSOR_CO_1,NULL as QA_3SB_03_SENSOR_CO_1, iurd1.dbo.Analogicos_2SS.E3TimeStamp FROM iurd1.dbo.Analogicos_2SS)
    UNION ALL
    (SELECT NULL as QA_2SB_03_SENSOR_CO_1,iurd1.dbo.Analogicos_3SS.QA_3SB_03_SENSOR_CO_1 as QA_3SB_03_SENSOR_CO_1, iurd1.dbo.Analogicos_3SS.E3TimeStamp FROM iurd1.dbo.Analogicos_3SS)
) results
ORDER BY E3TimeStamp ASC