The big day is this week. Any last minute tips? Should I bring a cover letter to the 2nd int., or just my resume/references? Would kahkis with a button up and sweater be too casual to wear? How about a non-suit skirt (black with subtled black detailed pattern), button up white blouse and open-toed shoes? It's going to be so hot and I know that I'll also be seeing the first person I interviewed with, and will have to wear the same suit jacket (different shell underneath) if I go the suit route.
These questions are impossible to answer generically; every company is a bit different. But I would not bring a second cover letter if you already submitted one with your resume. Just bring a spare resume or two and your references.
If the people you interviewed with the first time were wearing suits, then you'd probably do well to dress formally even if it means using the same suit jacket you wore before.
I've worn a suit to interviews at several jobs where everyone was dressed casually, and while I stuck out like a sore thumb I still felt better being overdressed than under. Going back to my second interviews at those places, though, I was more casual.
It's funny, everyone's definition of "formal" and "casual" is different. I was invited once to a wedding and was told to wear informal attire, so I arrived wearing a nice shirt and trousers but no tie or suit, only to discover that all the men there were dressed in suits -- to them "informal" meant "no tuxedo" but you were still expected to be wearing a suit.