This might not help your situation much, but when i was doing night trains with a companion, when making the reservations in person we had no troubles requesting a couchette where we were always in the same cabin together. We had a Railpass so i wasn't worried about the cheaper fare for the advanced purchase aspect, as all we had to pay was the extra fee for the couchette/reservation, and not the full fare. We just went to the rail station the day before (or sometimes the day of) departure and requested being in the same cabin with no troubles. As I had a railpass, i don't know if it is difficult to request the same cabin or not, as i have never booked a ticket over the internet.