Saving from List to txt

There’s a handy little method File.WriteAllLines — no need to open a StreamWriter yourself:

In .net 4:

File.WriteAllLines(speichern, ausgabeListe);

In .net 3.5:

File.WriteAllLines(speichern, ausgabeListe.ToArray());

Likewise, you could replace your reading logic with File.ReadAllLines, which returns an array of strings (use ToList() on that if you want a List<string>).

So, in fact, your complete code could be reduced to:

// Input
List<String> data = File.ReadAllLines(pfad + datei)
    .Concat(File.ReadAllLines(pfad2 + datei2))
    .Distinct().ToList();

// Processing
data.Sort(); 

// Output
data.ForEach(Console.WriteLine); 
File.WriteAllLines(speichern, data);