>Why would not you post the actual UPDATE command (you can disguise column names if you want) and then it will be easier to give you the code. In any case, I don't see why the suggested approach is not going to work. You can always split one command into 2 using CTE (common table expression).

UPDATE Client
 SET DayBetweenFirstDateSecondDate=
  CASE
   WHEN Client.FirstDate>'1899-12-30 00:00:00.000' AND
    Client.SecondDate>'1899-12-30 00:00:00.000' AND
    Client.SecondDate>=Client.FirstDate AND
    Client.NoCompany>0 THEN
    (SELECT COALESCE(SUM(Temp.[Second])/3600.0/8,0)
    FROM (SELECT DATEDIFF(SECOND,
     CASE WHEN BusinessTime.Start>Client.FirstDate THEN BusinessTime.Start ELSE Client.FirstDate END,
     CASE WHEN BusinessTime.[End]<Client.SecondDate THEN BusinessTime.[End] ELSE Client.SecondDate END) AS [Second]
     FROM BusinessTime
     WHERE BusinessTime.NoProvince=
      CASE
       WHEN (SELECT NoCompany.NoCountry FROM NoCompany WHERE NoCompany.Numero=Client.NoCompany)=1 THEN
       (SELECT NoCompany.NoProvince FROM NoCompany WHERE NoCompany.Numero=Client.NoCompany) ELSE 84
      END AND
      BusinessTime.NoHoliday=0 AND
      BusinessTime.Start<Client.SecondDate AND
      BusinessTime.[End]>=Client.FirstDate) Temp)
   ELSE 0
  END