Can you buy all 4 berths in overnight Couchette for 2 travelers?

First off, I would suck it up and enjoy the ride. However, you can buy all four berths and no one will say anything.

I'm sure you can buy all four berths if you buy "Global" tickets. Night train tickets have two parts, the rail part and the accommodation part. They sell complete (Global) tickets for those without the rail part and just reservations (accommodation surcharge only) for those who already have the rail part (purchased separately, annual pass, rail pass). I don't think they would let you buy just reservations. From RailEurope for purchase of a passholder reservation: "A Passholder fare can be purchased in place of the full fare and is valid on the City Night Line train provided the pass covers the Entire journey."

As a young woman I've never had an worries traveling in a 6 person "mixed company" couchette compartment. I'm in my 50's now and still would not worry about it. The doors to the compartments lock, and the person sleeping nearest would lock it.

There is no reason to be concerned about safety. And meeting other travelers can be part of the experience. But if you truly do not wish to share a couchette, you'd have to buy all 4 tickets. As someone else said, you must buy the rail and berth tickets, not just the berth reservations. Once a berth is reserved, it is not resold but a conductor could ask you to allow someone else to join you if other cabins are really crowded. It'd be in your rights to refuse but rather rude and accepting would be a waste of your money.

Thanks everyone. I feel much better about the couchette and we've decided to go for the experience, hopefully sharing the ride with some friendly and interesting travelers.