You can also just take different seats when you are on that train. Most of the seats are not reservable on that train anyway, and you can sit in any unreserved seat you want.
Those trains are 10 cars long, and seats are only reservable in 2 cars (and the restaurant). As a result those people who mistakenly thought that seat reservations were needed on that train (they are not) end up all sitting close together, and yes, sometimes it is not possible to find two seats together in the reserved section anymore.
Be at the train platform 10 minutes before departure so you are there when the train pulls in. Then you will have the pick of seats, and can look for two next or opposite each other on the right side of the train.