How to sync different Photos libraries to different iPhones via iTunes?

We have two iPhones, and we each had our own iPhoto libraries of our respective pictures. ITunes always was able to sync the correct iPhoto library with the correct iPhone.

Now with Photos and the new OS, I can't figure out how to get iTunes to recognize multiple Photos libraries. It only recognizes one (the first one i converted to Photos) - no matter which phone is plugged in. Suggestions?