SELECT datetime(Table1.stamp), Table1.value, Table2.value
  FROM (SELECT Table1.stamp AS stamp1, max(Table2.stamp) AS stamp2
          FROM Table1
               LEFT JOIN Table2 ON Table1.stamp >= Table2.stamp
      GROUP BY Table1.stamp)
       LEFT JOIN Table1 ON Table1.stamp = stamp1
       LEFT JOIN Table2 ON Table2.stamp = stamp2;