You can get a code for each spg account. I haven't tried making a new one, but open a new account and im sure you can get another $15 as well..
You cannot combined them, but maybe if you can buy a gift card with it and combined it that way.
It works for merchandise (t-shirts etc.) from their store. I haven't found a way for free shipping though.. usually about $5-$6 and order ($15 worth)