These have been outside in full sun for about 2 months, ever since temperature has been sufficient in NY. I feed them every 2 weeks with BR-61. The one on the right bloomed from 2/3 of the branches last year, each of which has pushed out 3 new branches. It appears like the other branch is showing beginning signs of an inflo, but hard to tell. Is this normal/to be expected? Plumeria blooming from the branch that did not previously bloom?

The plumeria on the left is a Tahitian sunset that I bought this year, so hoping for a first year bloom!

Generally it is more likely for a Plumeria to bloom from a stem that didn't bloom before.
Where you live, your season is very short. I would consider supply extra light during the early season to increase your chances of success.

When that Tahitian Sunset blooms you will LOVE it. Mine is now really tall so I generally just see the bouquets up high, and some blooms when they fall off. But it has always been a favorite of mine.