If you already have a busy Photos library, you’re going to be working with two different image libraries now, which is why it’s generally best to allow Photos app to pull in your i Photo or Aperture library on first launch.
If you’d like to merge two different libraries, well, at the moment, there is no way to directly perform a merging of an i Photo library with a Photos library without manually intervening and importing photos on your own.
The great thing about this app is the way it allows you to divide libraries into different aspects or themes.
So, for example, you can have one folder for work images and one for personal photos on different Macs.
Once the images are downloaded to your Mac, the flash storage device is more than likely erased to make room for a new batch of photos.
If you’re an avid photographer, your image library has the potential to be bursting at the seams with images you've taken over the years.
This way you can open Aperture and i Photo libraries while using any of the two applications.
To avoid loss of important data, you should backup the libraries before the process of merging.Modern photography has done away with the old-fashioned film negative or slide, which served as very good methods of archival storage of images.