I had been wondering about this, too, but with the current model, they would show as duplicates, because all partners' children show up once as a child of the partner, and if their secondary parent is set to the step-parent, then they will appear as a child of the step-parent. And adding the biological parent as part of the step-parents would make them duplicate for the biological parent as well (oddly enough it sets two children who branch out from the biological parents' partnership instead of one of the duplicates show up which branch out from the step-parent's partnership, so this may be a bug). Also, in the stepchild's view, for the first scenario, you'll be able to see parent view properly. For the second scenario, the biological parent will show up twice, first as a partner of the other biological parent and then a partner of the step-parent. But with both of these, you won't be able to see any half-siblings if they have any, which is very odd. But you can do this if they do have no half-siblings for a proper tree view from the stepchild, otherwise you would probably have to leave it as spouse's child instead of stepchild for now. This also applies if you are doing other parent types, although step would be the correct type to use.
I am not really sure how to get around this, but Family Echo should probably make a check to see if there are any duplicates of children and parents, and try to delete one of the duplicates if possible.
