How can we offer a plenary indulgence for the souls in purgatory and still keep our own plenary indulgence?

I do not think the Church says you give a plenary indulgence to a person in purgatory; that is where the mistake is. Plenary indulgences are designated for the person who performs them, because you can gain indulgences on earth but you cannot gain them in purgatory. So the confusion comes from thinking of it as handing your indulgence over to someone else and losing it yourself. The indulgence is gained by the one who does the work here on earth.