If you’re currently travelling, go to a ticket office, preferably in a large station and book there.

A double sleeper is a coupé with 2 beds in a sleeper coach. If you book for 2 people, you should be placed in the same coupé, if possible.

If you want seats, they are normally placed next to each other if possible. In that case, you can book online via Czech railways. Specify 2 adults with interrail and click Search. The night train will show up as "Free”:

Click on the green button and then on Continue, and the price should appear as CZK 154 (~EUR 6.32) for 2 seats. Then click “Place in shopping cart” and you'll already see the assigned coach and seat numbers. Fill in your name and email and then pay. You will receive a PDF and it's best to print that.

For seats: very often they do not even check that-they will want to see the ticket=your pass. That is if you sit on the assigned seats. And that will happen again in Slovakia and again in POland-in the middle of the night. If you are in Bp for a few days before that-just do it at the Keleti station and pay with HUF.

For beds (there are maybe also cheaper couchette=liegewagen=more hoStel like basic beds- there will be an attendant in ecery such car and most often they will want to have some kind of paper for their booking list.

You must also check what SK=Slovak wants from you as they have a covid-system of prenotification online and there are reports they check that at borders! Reunite EUrope is best site for these things and links