Can I merge two Google Photos accounts or transfer from an old account to a new one?

I suggest going through the following article to know about the different methods one can use to transfer pics from one account to other.

 

How to transfer photos from one Google account to another,

 

The article offers several tips on how to do so

However, method C would serve the purpose.

That's quite a complex situation, because if you change the backupsettings in the Google Photos app to your new account, all what is still present on your current Android-phone will get backupped into your new account - but that process can take weeks if many videos are included. And you also may have older photos and videos in your old account which are not present on your current phone: check on photos.google.com in the Chrome-app while being logged-in with your old account (you also can do that on your computer). 

So that is not the right method, so it is better to use the temporary partnersharing method between your accounts: see transfer (method C). 

 

But I have to think about how this all should be done safely on one phone only, because you also need to remove the content on your phone to prevent backupping into your new account. 

 

Please reply that you have read this - else it makes no sense for me to go on.