WITH sens1 AS (
  SELECT '2020-05-02 10:00' dt, 123 v
  UNION ALL SELECT '2020-05-03 11:00' dt, 458 v
  UNION ALL SELECT '2020-05-04 12:00' dt, 789 v
)
, sens2 AS (
  SELECT '2020-05-02 10:30' dt, 111 v
  UNION ALL SELECT '2020-05-03 11:30' dt, 222 v
  UNION ALL SELECT '2020-05-04 12:30' dt, 333 v
)
, q AS (
  SELECT dt, v sens1, NULL sens2
  FROM sens1
  UNION ALL
  SELECT dt, NULL sens1, v sens2
  FROM sens2
)
, q2 AS (
  SELECT dt, sens1, sens2
  , SUM(IF(sens1 IS NOT NULL, 1, 0)) OVER (ORDER BY dt) gch1
  , SUM(IF(sens2 IS NOT NULL, 1, 0)) OVER (ORDER BY dt) gch2
  FROM q
  ORDER BY dt
)
, q3 AS (
  SELECT q2.*
  , MAX(sens1) OVER (PARTITION BY gch1) sens1_f
  , MAX(sens2) OVER (PARTITION BY gch2) sens2_f
  FROM q2
  ORDER BY dt
)
SELECT -- *
dt, sens1_f, sens2_f
FROM q3