Both hermit crabs are alive.
As David figured out with his superior google-fu, the thing I flushed that looked like a dismembered crab being eaten was the husk of Spartacus, who had moulted. They eat their own discarded husk for calcium to help them recover from the process.
The crab I thought was Spartacus in a new shell was actually Crixus, who sensed vulnerability and was trying to kill the weakened Spartacus. He wasn't crying victory by harassing what I thought was an empty shell and chirping, he was engaging in a shell fight. Spartacus, weak from moulting, was in no shape for a challenge and had managed to retreat so far into his little shell and Crixus had managed to push enough sand into the shell that it looked empty.
For a time, Crixus had both shells completely locked together at the opening. I thought he was being greedy and trying to live in both shells simultaneously, but he was actually trying to kill Spartacus. When he had disengaged the shells, I ended up removing the apparently empty Spartacus shell from the tank to see if Crixus would calm down. A while later I went back to check and what I thought had been an empty shell had legs and was walking across the desk. It was very confusing.
I put Spartacus in an old fish bowl with some new sand to recover, and he seems to be fine. He was very thirsty and hungry. I feel terrible that he had to spend all day with a murderous tank mate.
I'm relieved that Spartacus, my favourite of the two, isn't a murderous violent cannibal.