Q: How to avoid two large photo libraries?

Deleting is just like with any other file.  You select the Photos Library.photoslibrary in Finder, right click and Trash.  Then empty trash to free the space.  Before you do, you will want to make sure Photos doesn't have pics you took since the conversion because iPhoto shouldn't have pics take after the Photo activation.

Also, for benefits of Photos over iPhoto, please check this thread.  What is the advantage of Photos over iPhoto? I can't seem to find any.  In fact, Photos seems vastly inferior

Please be advised, iPhoto is no longer being supported by Apple.  Nor Aperture.  Over time, these items will remain stagnant while Photos will continue to gain features.

Does anyone know why I have two large photo libraries?  I've recently upgraded to El Capitan on my Macbook Pro and went along with Photos indexing my images even though I had updated my iPhoto library after the install.  Now I have an iPhoto library that's 69Gb and a Photos library that's 65Gb.

If your Photos Library is a library that has been created by migrating your iPhoto Library, the two libraries together are using much less storage than the added sizes of the two libraries. The original image files and the previews are hard linked files. They are shown as separate files but are stored in the same location on your hard drive, so they are using the storage only once.

See these documents:  Photos saves disk space by sharing images with your iPhoto or Aperture libraries - Apple Support

and  Six Colors: The (hard) link between Photos and iPhoto

Deleting one of your photo libraries will not free much storage. 

I'm still using iPhoto for calendars etc so I'd like to know how to get rid of the much needed 65Gb being taken up by Photos.

Apple is dropping the print products support for iPhoto.  After the end of the month you will no longer be able to order books, calendars, or prints using iPhoto or Aperture.  Photos for Mac will be the only way to order calendars from Apple.

You can continue to use iPhoto right now, but this software is no longer supported by Apple and may stop working at any time.

Wwhen you migrated your iPhoto Library to Photos a new Photos Library was created,  it it does not duplicate your iPhoto images. Rather, the same images are hard linked to both libraries. Finder reports the size of each as a complete Library because each one has access to all images. If you delete the Photos Library, you will not gain any significant space on your drive.

As of now, whatever changes you make in iPhotos are not synchronized to Photos and vise versa. The two are completely separate.

To summarize...

Yes, you can delete the Photos Library file, but you will not gain much space.

It is a bad idea to use both, iPhoto and Photos; they are not synced.

You will be able to migrate to Photos when iPhoto stops working.

Personally, I would advice to buy the bullet and switch to Photos now. It's the future! By sticking with iPhoto, you're just delaying the inevitable.

It would have been great for Apple to notify anyone who orders regularly from iPhoto!

At least Apple included the warning, when photo books have been delivered recently.  That is how we know it. 

The Print Products page is only listing Photos for Mac products:  http://www.apple.com/osx/print-products/

while the Product Line Details page is still showing iPhoto:  Apple Photo Services: Product line details for books, cards, and calendars