I had initially just requested one, but the FA insisted I have two.
It's comparable to Economy Plus on United or Main Cabin Extra on American; it's not comparable to real "premium economy" on airlines like British Airways, Virgin Atlantic or Air France.
This has been a more recent development as the flight was previously operated out of Terminal 3 departures given its status as a OneWorld member carrier.
Seats do not recline—which actually might help a little—but the seating is still extremely tight.
airberlin also has codeshare agreements with more than 20 airlines.
In total, you could easily end up waiting an hour or more in all the various lines at Dusseldorf Airport, so be sure to arrive with plenty of time.