Note to the mods, I couldn't decide if this was more appropriate in "The Wider Picture" or in here, so feel free to move if needed

I am looking to take the Spanish Sleeper Service from Barcelona - Granada around December time... I have a connection to make in Granada for Almeria, I plan to catch 21:29 Trenhotel service off Barcelona, connecting on the 10:07 off Granada - Almeria Local Service. (Despite these places being 2 large cities, there's only 4 trains each way a day

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I was planning on booking a Berth, but I have conflicting information regarding the sleeper berths. Sharing a berth with a strange foreign person is not an option!
On the website it gives me the option to book Preferente Berth (Single/Twin) or Gran Clase (Single/Twin with ensuite). Now how do I book just a single/my own berth on the Renfe website? Is there an option where you can specify that, or do I book 2 tickets?? If it ment that I had a berth to myself, it would be worth buying 2 tickets!!
Does anyone have experience with the Spanish Sleepers??
Thanks

TT