Sharing photos between two accounts
Me and my wife use Picspro android app for Google photos.  It allows browser authentification to sign into any amount of Google Photo accounts without having to add the full Google account to the phone itself.  Personally,  We keep all our photos on her account,  using this app I can be logged in and view them and also upload to it so our photo collection is unified.  We also use this to upload to our Mum's accounts too and it works a treat. Settings>accounts>add account.  Untick the box next to "Use phones account" 

To fully back up all your Google photos to your PC, just go to Google Takeout and you can download any or all of your photos whenever you like. 

Hope this helps :)