I need to merge 2 iCloud accounts into one

Merging Apple accounts (IDs) or transferring data is not officially supported, nor can it be done easily or completely.

Apr. 2018 post by ASC helper Winston Churchill on transferring data from one Apple ID to another. https://discussions.apple.com/message/33333380#message33333380

Feb. 2021 tips from Eric Root https://discussions.apple.com/thread/252468781?answerId=254653802022#254653802022

Look into using Family Sharing to continue to be able to use items purchased with one Apple ID with other Apple IDs.

Family Sharing - https://support.apple.com/HT201060

Set up Family Sharing - https://support.apple.com/HT201088

You cannot. There is no way to merge accounts. The best you can do is move content like files and photos manually from one account to another.

Purchases such as movies, books, TV shows and apps cannot be moved and would need to be bought again on the other account.

Outside of that you can use Family Sharing to invite one Apple ID so purchased content can be accessed from the other Apple ID. iCloud backups, files and photos would not be directly accessible using Family Sharing however.