icloud

Only Using Mac (signed with Apple ID B credentials)

1. Log in to https://www.icloud.com on a web browser with Apple ID A credentials. Download all Photos to a folder on Mac / PC
2. Open Photos App
3. From the Menu > File > Import > [Select the Folder in which the Photos from iCloud A are downloaded]
4. Click Review for Import
5. Click on Import all New Items
6. On Photos App
7. Photos > Preferences… > Check iCloud Photos.

All your iCloud A and iCloud B photos are in one place iCloud B & Mac Photos App

Only using iPhone (Assuming iPhone has enough space). Cumbersome process.

1. iPhone signed in with iCloud A (Apple ID A)
2. On your iPhone, iPad, or iPod touch, go to Settings > [your name] > iCloud > Photos, then turn on iCloud Photos.
3. Also Check Download and Keep Originals
4. On your iPhone, iPad, or iPod touch, go to Settings > [your name]
5. Sign out (This will turn off Find My)
6. Prompted for Apple ID A password - Provide
7. Prompt for Keep a Copy of your Data on this iPhone. Enable which ever you want.
8. Sign Out
9. iCloud A Data will be removed from the device. Which ever you had disabled in Step 7
10. Sign in with Apple ID B
11. Again on your iPhone, iPad, or iPod touch, go to Settings > [your name] > iCloud > Photos, then turn on iCloud Photos.
12. Also Check Download and Keep Originals

You can see if photos or videos has uploaded to iCloud when you open the Photos app and view your library in Moments. Now all your Photos are on iCloud and iPhone

It is lot easier on a Mac, though is feasible on PC too. Assuming from iCloud A to iCloud B assuming both has few photos.

1. Log in to https://www.icloud.com with Apple ID A credentials. Download all Photos to a folder on Mac / PC
2. Transfer all downloaded photos from the folder to iPhone using AirDrop (Not for PC) which is signed in with Apple ID B account (On an iPhone)
3. Once Transferred, all the photos are in one place and it will now automatically sync to iCloud B (*-If you had turned on iCloud Photos, all your content will automatically get stored in iCloud B )