Magnifica does not usually split like that. The do split with stress.
When my Magnificas split, several time over the years, they start to look bad, mouth dilated, then a hole developed on the foot. First sign cause me to know the split is for real when I start to see rock though the Magnifica's mouth. From there the hole get bigger and the side start to move apart. The Mag start to have a "O" shape, The next step happen when the "O" turn to a "C" when one of the side break. Then the other side break and there are two Magnifica without mouth. Then the two break started to heal and then there are two whole smaller Magnifica. I documented this in an old post on RC around 2002 or so. The server upgrade lost it and I was not able to find the old Photos.
Maybe somebody can document the whole process again. The next time my Magnifica split, I will try to document it.