How to use 2nd photos library with 2nd iCloud account
My father just started using his iPhone and iCloud account to synchronise his photos. Some are on his hard drive on a Windows computer and some are in iCloud (the more recent ones taken with his iPhone), and some are still in Google Photos. I would like to get all of them in order for him.
I have a Mac and would like to create a 2nd Photos library in which I can add all his photos and organise them for him by correcting dates, adding geo tags, creating folders, etc... I have created a second Photos library.
My question is, is how I can add my dad's iCloud account to my Mac (once I'm done with his library and all the photos) and sync them to his iCloud account? This will be a one time sync after which he can keep it up to date on his iOS devices.
I hope I have explained my intentions correctly and any advice would be highly appreciated.