how to join two different icloud accounts

Not easily and not 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

2accounts1device Said:

"how to join two different icloud accounts: I have 2 icloud accounts and 1 device. can I condense accounts ?"

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iTunes is of No Option:

As of this reply, iTunes purchases are non nontransferable between Apple IDs. This is all due to licensing between Apple and the Music/Movie studio.

Well, as you can see the official preferred solution according to Apple is a flat out NO, but there is a can-do solution because what you actually asked about involves a lot more than just iTunes purchases (which can be shared from another account by methods that are ones that Apple itself offers up, so ???). ;-)