I'm not sure reusing them like that would be a god idea. The microfleece inner isnt waterproof, it still gets wet it just is no way near absorbant as a natural fabric which is why it has a "stay dry" effect. It would retain a bit of urine and I imagine would get a bit smelly if it was used over 2 - 3 changes.
Did someone at the store tell you to do that?