Hi JR.
I did this trip last year. Yes, you will need Kuna for Croatia, and I took just a small amount of Euro's for Montenegro, as you are only there for a short period of time, and will probably only need it for a few souvenirs and a bit of lunch. That's presuming you are just doing the day excursion?
As a word of warning, I have found that most banks (I use Tesco bank), don't carry Kuna in stock, and you will need to order in advance. There are a couple of ATM's in Cavtat village, and also a Post Office.
JY have changed the hotel this year, and the itinerary for this holiday is excellent. I am sure you will have a great time. If there is anything you want to know I am happy to answer if I can. 🙂