You need to set up two different postage discounts.

One to make all additional items have 0 postage, no matter if they are the same item or different items, and one to give the buyer free postage if they spend $10 or more.

The first one has to be a flat rate postage discount. The second one has to be a promotional postage discount, then you need to apply both of them to all existing listings. (They won't apply automatically). 

If you need more specific help on how to set it up, I can take you through it step by step. 🙂