They weren't copied. The two libraries initially share the same original files on the HD via hard links so they don't take up twice the amount of space on the hard drive. They can be copied/backed up independently.
New photos added to either library are independent of the other library.
Here's more info on hard links:
Photos saves disk space by sharing images with your iPhoto or Aperture libraries - Apple Support
The (hard) link between Photos and iPhoto
If you delete the iPhoto library you won't recover the full size of the library. Only the difference between the Photos library and iPhoto library sizes.
You can still use iPhoto if you had some version of iPhoto 9.x prior to the upgrade to el Capitan if you follow the directions posted by Tererence Devlin in this post:
Terence Devlin
Apr 14, 2015 11:21 AM
Re: Is Iphoto gone ? i want it back!
in response to Johannes666
Recommended
Go to the App Store and check out the Purchases List. If iPhoto is there then it will be v9.6.1
If iPhoto 9.6.1 is there, close the App Store, drag your existing iPhoto app (not the library, just the app) to the trash and then go back to the App Store.
Install the App from the App Store.
Sometimes iPhoto is not visible on the Purchases List. it may be hidden. See this article for details on how to unhide it.
http://support.apple.com/kb/HT4928
One question often asked: Will I lose my Photos if I reinstall?
iPhoto the application and the iPhoto Library are two different parts of the iPhoto programme. So, reinstalling the app should not affect the Library. BUT you should always have a back up before doing this kind of work. Always.
NOTE: the key to this procedure is to move the existing version of iPhoto 9 to the Trash bin in the Dock (don't empty yet) before going to the App Store.
