Can I transfer photos and videos from one iCloud account to another?

You can SHARE between different accounts through "Family Sharing." Not sure this will accomplish what you want, but it might. You could add both accounts to a "family" to do this.

See https://support.apple.com/guide/mac-help/family-sharing-share-photos-videos-mac-mh869cba7026/10.15/mac/10.15

You could export the photos then import into the other.

I don't know if there is another way, and I have no way to test it out.