Intuitively I would say that I were locked together in a cell with some other inmate, i would be a much worse fate than if I were locked in solitary confinement. Being locked in solitary confinement is surely not a nice fate, but as Sartre expressed it: "Hell is other people". In this sense I pretty surely would consider being locked with another inmate in a cell as the real "hell on earth".
So what is so bad about solitary confinement in comparison to non-solitary confinement? As I said: If I were to choose, I would immediately choose solitary confinement by far.