I will also have my 2nd round in Germany soon (maybe we could get in touch).
As I know from friends that work at BCG, the real-life interviews will also test the weaknesses that you showed in the first round.
It can range from:
- getting information from a client during a taxi ride
- producing an executive slide for a partner based on multiple slides you receive
- convincing colleagues over lunch about an idea
- writing an email to a client
Hope it helped a little bit. BCG says you can't prepare for those short interviews.
I think you can already go through some situations and train things like producing slides etc.