The only reason to use two magnets is if the effect is too much stronger in the center of the chest. Magnet zones have a "falloff" which is what makes the effect smooth and organic but that also means the effect is strongest in the middle of the zone. So if the chest center starts to look pinched you'd want to use 2 magnets. There's a way to edit the falloff for a magnet zone but the effect will still always be strongest in the center. An example would be using a magnet to narrow an abdomen, if you don't want a slot bellybutton you'd use 2 magnets positioned on the sides instead of one in the middle. Hope that makes sense.

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Annie Dillard