ハンガリー鉄道の回答です

Dear Passenger,
Thank you for your inquiry.
We would like to inform you, that in our international ticket purchase system you are not able to chose your seats, the system gives the seats automatically according to the free places on the train. Usually the seats are reserved together, if they have been purchased together.
Thank you for oyur understanding.
Best Regards,
Endre Zsadányi
Customer Service Assistant
MÁV-START Railway Passenger Transport Co.
Customer Service - MÁVDIREKT