Q: Photos and iPhoto

No Photos replaced nothing - it is a totally new program

Do you need both? Only you know that. Generally it is suggested to keep both for a t least a few months until you are sure of what you are going to be using

their libraries do not duplicate photos but use hard links to share access to the original photos and previews - Photos saves disk space by sharing images with your iPhoto or Aperture libraries - Apple Support - so delete on library saves very little space

LN

I thought Photos replaced iPhotos but I seem to have both. Do I need both? Can anyone help please?

Photos could not migrate everything. To be able to use your books, calendars, cards you need to open the library in iPhoto.

And some things simply cannot be done in Photos - just compare the functionality - see Old Toad's User Tip:

  Photos vs iPhoto:  Features and Capabilities

iPhoto is  still the more versatile application; I simply would not burn the bridges back to iPhoto until you are sure, that each and every photo migrated correctly and you are sure, that Photos will suffice for your needs.

I need to get back into iPhoto, because I had more than one iPhoto Library of photos, but iPhoto won't open.  How to I get access to the photos that were in a separate Library?  All of my genealogy photos were kept in a separate library from my main library.  Once I do get access, what is the best way to move them to Photos and keep all of their titles, dates, descriptions, keywords, faces...

I n the future please do NOT ThreadJack existing threads on a totally different topic - start your own - ThreadJackin confuses you, the original poster and the people trying to help

To migrate additional libraries hold down the option while launching Photos and select the library you wish to open and migrate

LN

If you need to keep iPhoto working, update it to version 9.6.1.  You can do that by reinstalling from the AppStore.  Follow carefully the directions in this User Tip:

Get iPhoto 9.6.1 if you didn't update before OS... | Apple Support Communities

If you just want to open a different iPhoto library in Photos drag the iPhoto Library onto the Photos icon in the Dock. That will create a converted copy.

You can neither merge libraries in iPhoto nor in Photos.  iPhoto Library Manager can be used to merge libraries, but it is not free:  Fat Cat Software – iPhoto Library Manager

If I want to move photos from Photo to iPhoto is there an easy way to do it please.

How many photos do you need to move?

It would be best to start by opening your old iPhoto Library.  The migration from iPhoto to Photos did not delete your old iPhoto library but just renamed the filename extension to ".migratedphotolibrary". You can open this library still in iPhoto, if you click the "Open in iPhoto" button, when you are seeing a warning that the library has been migrated.

To move your newer photos from Photos to iPhoto use the "All Photos" album. This is sorted descending by the date you added photos to your Photos library. Your most recent photos will be at the bottom of this album. Select all newer photos and use the commend "File > Export > Unmodified Originals" to export the photo to a folder. Then import them to iPhoto from this folder.

It is better in some ways but worse in others.

Apple is working on a new version of Photos. I am sure it will improve during the years to come. While we wait it is good to still have iPhoto.

See the Preview:  Apple — OS X El Capitan

The next version - Photos 2, will bring the ability to use external editors via app extensions, better spring options, geocoding, and more.