Sequential Multi-Threading

Question

I wanna build Multi-Threaded procedure for the following code, but not able to correctly build it. I tried using Semaphore to sequentially complete the code but it only executed the myStream2.CopyFrom(myStream1, StrToInt64('$' + SL1[(i - 1)])); part which comes under else part. The problem presents is that if we restore those streams into output filestream (mystream2) haphazardly, the file will corrupt.

Can anyone help me please.

  for i := 1 to SL1.Count do
  begin
    if Length(SL1[i - 1]) > 12 then
    begin
      TempDir := SrcDir + 'temp_' +
              IntToHex(Random(2147483647) + Random(2147483647), 8) + '\';
      ForceDirectories(TempDir);
      for k := 1 to 2 do
      begin
        tempfile := TempDir + IntToHex((i - 1), 8) + '.tmp' + IntToStr(k);
        fs := TFileStream.Create(tempfile, fmCreate or fmShareExclusive);
        fs.CopyFrom(myStream1, StrToInt64('$' + DecodeRefStr(SL1[(i - 1)])[k]));
        fs.Free;
      end;
      ExecAndWait(SrcDir + 'process.exe', Method + ' ' + '"' + TempDir +
                  IntToHex((i - 1), 8) + '.tmp1' + '" "' + TempDir +
                  IntToHex((i - 1), 8) + '.tmp2' + '" "' + TempDir +
                  IntToHex((i - 1), 8) + '.tmp4' + '"', TempDir, 
                  True,  True);
      tempfile := TempDir + IntToHex((i - 1), 8) + '.tmp4';
      fs := TFileStream.Create(tempfile, fmOpenRead);
      myStream2.CopyFrom(fs, 0);
      fs.Free;
      TDirectory.Delete(TempDir, True);
    end
    else
      myStream2.CopyFrom(myStream1, StrToInt64('$' + SL1[(i - 1)]));
    end;
    myStream1.Free;
    myStream2.Free;