If two different people buy your body and your soul, these items will be separated, so the lyrics simply suggest that it's best to keep them together. And considering the reference to missiles, "mushrooms" will presumably mean mushroom clouds - nuclear missiles make mushroom clouds, don't they ? Anyway, since when did song lyrics have to make sense ? The most successful songs often seem to make very little sense. Besides, in a confusing world, confusing lyrics are realistic, I think.