Well, that was interesting. I should've planned better.
So I put the crab in, right. He's sitting on one of the plants... And the betta charges at him. He snaps a claw at the betta, barely missing. The betta's two.. long skinny fins on his body look somewhat like crab legs, and I think he's severed some of the crab's appendages. So I frantically pull off the top of the tank, grabbing the net at the same time, and shove it in there to just break them up. The crab takes off. Crabs are harder to catch in a hexagonal tank,
btw. I finally get him and place him in his container, but realize I should clean his water. So I go get the little cup my betta came in, and put the crab in there. I go outside and start hosing the tank (don't want the solid debris in the sink) and discover that he DIDN'T eat his old shell. He broke it into pieces, and buried it in the gravel. Go figure.
Finally put water in it, dechlorinate, and put the crab back. Then replace the top of the betta's tank.
In short, don't try it unless you've planned well. >.>