Thanks for asking the question. It has been on my mind for a while. I once had a double cabin to myself and there was only one safe. So something tells me that it there will only be one safe even if they are two independent (solo) travellers in that one cabin.
Also... does anyone know whether the pursers office will run two separate tabs for one room.
There will only be one safe, but you can use the safety deposit box at the purser's (information) desk. They are rather small, and they encourage you to use your room safe, but if you just briefly explain your situation, they will let you use the safety deopsit box. The room safe operates on the last card swiped through, so whoever locked it last must unlock it. This is why Carnival specifically recomends not using your "Sail and Sign" card for the safe key since the key you open your door with would be the one that opens the safe, too I personally recomend using a grocery store shopper card or somehting without your name on it for the key.
There is only one safe in the cabin, so if you are sharing a cabin with a stranger, I suggest that you put your valuables in a safe deposit box in the purser's office. You not only have to be wary of someone stealing from you, but also that YOU might be accused of stealing from THEM. So let them have the safe in the cabin. Sorry to sound so paranoid, but experience makes me so. :-))