Originally Posted by
Xandrios
[...]So I'm guessing that the A3 website somehow doesn't properly handle having two etickets.[...]
This seems to be indeed the case. I was recently booked on a multi-city trip (ARN-MUC-SFO-JFK-FRA-ATH), all LH-operated apart from the US domestic one (UA) and the last one (A3). Although I was able to reserve a seat in all of them, including the UA flight, this was not the case with the A3 one. When I attempted to do this through A3's website, the usual "Add seat" option did not show up at all. It was moreover stated that I have no free checked bags (more precisely, a hyphen appeared under "baggage allowance"). On top there was a message in red informing me that adding additional luggage was not available online, so I should call them...
Perhaps non-compatible booking engines?