No, I haven't. I don't know how the split happened. If the starfish grabbed it, I would imagine it not letting it go until it was gone. A crab wouldn't be able to do that either IMO
Those serpent and brittle starfish hide pretty well and only come out when a meal is available, or after dark, searching for a meal.
I have seen the green brittle stars perched up like a tent inside a cavern, waiting for a fish to swim under. They fall onto it and wrap it up and chow down. Even if this happened, ripping it in half would be hard to imagine.