Don't try to make Twin Falls a first stop as part of Hana Highway day, but rather branch out from exploring around Paia. Bring your suit and camera. At the tallest, upper falls, daredevil school kids and game visitors leap from on the the rock ledges. It's easy to miss some of the falls, since the main pathway misses a couple and other pools are upstream of a confluence. Take a look at Maui Trailblazer, page 119.
Taken as a whole, and considering access and crowd issues at other pools on Maui, and other islands, Twin Falls is the leading contender for Best Freshwater Swimming in Hawaii.