[opel431]

Попробуйте через потоки

Код:

procedure ConcatenateFiles(const InFileNames: array of string;
  const OutFileName: string);
var
  i: Integer;
  InStream, OutStream: TFileStream;
begin
  OutStream := TFileStream.Create(OutFileName, fmCreate);
  try
    for i := 0 to high(InFileNames) do
    begin
      InStream := TFileStream.Create(InFileNames[i], fmOpenRead);
      try
        OutStream.CopyFrom(InStream, InStream.Size);
      finally
        InStream.Free;
      end;
    end;
  finally
    OutStream.Free;
  end;
end;

ConcatenateFiles(['Part1', 'Part2'], 'OutFile')