I end up with two Libraries when migrated to Photos.

Actually there is just one library. There are HARD LINKS from each database to the actual photo location. You can delete either library and the photos will still remain, BUT changes made in one program won't carry on into the other. Check http://sixcolors.com/post/2015/02/the-hard-link-between-photos-and-iphoto/ for a good analysis.

Yes, sort of. There is only one copy of your photos, but the database for each program takes up some room, but no where near the amount of the photos. I'll repeat that after you've imported, you can still use both programs, but the changes made in one won't appear in the other... beware!

Supposedly you can delete either database and the remaining one will work with its own program (you may want to rename the iPhotos db which was renamed during the migration). I have good backups (hopefully) so I may try to delete the new one, BUT I am not recommending anyone else try this...

I will keep track of how big the supposed Photos library is (somewhere over 100G), and how much my drive gains after the delete, if I do get up the courage to do it.

Here's definitive proof:
Photos library: 123.23G
iPhotos migrated library: 126.31G

Available on Drive: 691.05G

I deleted the Photos library and emptied the trash.

New Available on Drive: 694.70G

*AND* iPhoto works just great. I did rename the file, but don't think that makes any difference.

I'm sticking with iPhoto because I can't find an alternative that will handle my 100+G of photos and keep the events/albums I've already created. Maybe the next update of Photos will be better, sigh.