I am doing a PhD with concentration on continental philosophy in Hong Kong (my MPhil also had the same concentration too), but I am really interested in political theory and political science and often audit relevant courses. I am wondering whether it is possible to get another PhD of political science in Germany, if I focus on empirical political science research or political theory, after finishing my current one, provided my German is fluent enough.
You should have a look at the "Promotionsordnung" (~Regulations for PhD courses) of the specific universities. As far as I know, it might not be possible to get the same PhD degree twice. They have to be from different fields. It should be possible to get two different PhDs as I know some people who achieved this in Germany.
If you already have a PhD and want to go through that process again in Germany, then you can instead try to get a PostDoc position that allows you to write your habilitation. Within the German context that would make more sense.