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The cabins are the same (Economy Single and Economy Double).

There are two fold-down bunk-beds in each cabin, so if two people occupy the cabin both bunks will be folded down at night; for one person, only the lower bunk will be folded down.

You've answered your own question: if you're a couple, then share a compartment (and save €120). Space is at a premium, but it's always an interesting experience.