Orderby Secondary Sort

Hi — I’m trying to add a secondary sort for a pod. This code works beautifully for a basic sort by unit:

[pods name=”podsname” limit=”-1″ template=”SA Listing” orderby=”unit”]

I would like to add a secondary sort of “last_name” and I’ve tried adding this in a variety of ways to the above with no luck. Perhaps there is a simple way and I just don’t have the syntax correct? I looked online and in the knowledge base with no luck. Thanks so much in advance for your help! Ann