Synchronizing Photos between two Google Accounts

I have an interesting situation where I need to copy photos and videos from one Google Drive account (not Google Photos) into a separate Google Photos account. Without going into the contextual details of why I need to do this, here are the primary objectives:

• Photos and videos are initially uploaded into Google Drive folders of account someone@domainA.com
• Photos and videos need to end up in Google Photos of account someone@domainB.com
• This is an ongoing activity (not a one-time activity)
• I would prefer not to keep multiple copies of all files on my local machine

From what I have been able to tell, Google does not provide a simple way to accomplish the above objectives. The only way to accomplish these objectives is to download from one account and upload into the other account. However, I do not really want to do that manually on an ongoing basis. 

The solution is based on Google's Backup and Sync application. The application already allows you to add multiple accounts - but they must target different folders on your machine. With that limitation, I would need to manually copy all files from one synced folder to the other synced folder (and that would violate the objective of not keeping multiple copies of the files on my local machine). The solution is to use two different Windows accounts on your computer. The Backup and Sync application does not run as a Windows service at the operating system level; instead, it runs as a startup script for the Windows profile (e.g. it is only running while you are logged in). 

Setup steps:

1. Make sure you have two different Windows accounts on your computer (e.g. one for each of the Google accounts that you want to sync.
2. Create a file system folder that is outside of your Windows profile; perhaps you might call it C:\GoogleDriveAndPhotos
3. Login to Windows as the first user (e.g. someone@domainA.com)
4. Install Google Backup and Sync
5. Add the someone@domainA.com account
6. On step 2 of account configuration, add the folder that you created in step 2 (that is where you want all of your My Drive files downloaded to).
7. On step 3 of account configuration, uncheck all options (you are not trying to upload anything from your computer into this Google Drive account)
8. Complete the configuration and let the sync process complete (may take a while, if you have a lot of files/photos)
9. Login to Windows as the second user (e.g. someone@domainB.com)
10. Install Google Backup and Sync
11. Add the someone@domainB.com account
12. On step 2 of account configuration, uncheck all of the folders (you are not trying to download anything from this account to your computer)
13. On step 3 of account configuration, choose the folder that you created in step 2 (that is where all of the photos/videos will be downloaded from the other account)
14. Choose the option to only upload photos/videos (not all files). (I also chose the option to upload as "High Quality", not "Original")
15. Complete the configuration and let the sync process complete (may take even longer, as upload speeds are often slower than download speeds)

Ongoing use:

1. Periodically login to the first Windows account and let the sync process download new photos/videos
2. Periodically login to the second Windows account and let the sync process upload new photos/videos