At Macworld, Glenn Fleishman reviews PowerPhotos 1.0, a companion utility for Photos for OS X that can find duplicates, easily manage multiple Photos libraries, and help migrate iPhoto and Aperture libraries to Photos for OS X.
The software does not merge Photos libraries; if you specifically want that feature, pick up iPhoto Library Manager by the same developer, Fat Cat Software. (According to the company’s blog, “This is something I’d like to add to PowerPhotos, though I can’t make any concrete promises, since doing so partially depends on whether Photos itself adds a few necessary enhancements.”)
Jeff Carlson | Never Enough Coffee Creations 