How to get Photos Library v2.0 content to Photos Library v1.5? I'm in the unenviable position of trying to merge multiple Photos libraries together. On an OS X El Capitan. 

Two of the Photos Libraries are version 2.0, so how do I merge their content into a v1.5 Library, so that my wife, who'll be using my OS X El Capitan -capable machine, will be able to continue with her photo escapades?

I'd hate to have to export the contents of v2.0 Photos Library to some  folder and then have to import it back, possibly with data loss. Is there some solution?
The problem: No way to open a Photo Library that is newer than 1.5 if you   are stuck in OS X El Capitan -land.

Solution: Inside of each Photos Library is a folder called Masters. You can access this even without having the newest version of Photos.



Start by right-clicking (ctrl-click if on touchpad) on the Photos Library and selecting Show Package Contents.

Follow that by going into the folder Masters. It will look somewhat like this:



Launch the Photos app and make sure you have that specific Photo Library open - and drag the contents of the Masters-folder onto your Photos -app. After a serious amount of waiting, the import process will start.