Can I book a long range flight for two passengers with different feeder-flights?

I, as a German living in Hamburg, am planning to book flights for a holiday in Panama.
I want to go there with a friend, who lives in a different part of Germany, near the border to Netherlands. The long range flight will probably be Amsterdam-Panama City (Either KLM or Air France).
What I need is something like:
- 1 Ticket Hamburg - Amsterdam - Panama
- 1 Ticket Köln - Amsterdam - Panama
However it is important that the long range tickets are registered as a pair by the airline. There are personal reasons for this, lets call them anxiety - I really want to make sure that, whatever happens (overbooking etc...), we both will definitly end up on the same long range flight. For all the options I consider, this has top priority.
As far as I can see, it is not possible to book this via the web interface. So the best options probably are
- go by car to the same location
- book the feeder flights indepentently.
I really dislike 1 because this means a long car trip after the flight, and 2 has the danger of delays resulting in a "no show" for the long flight.
Are there any options I am not aware of?
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