If you want to give all of Anatolia and Iraq to your younger brother, and you are a king, I think there is only one way to do so: You have to destroy one of the two kingdoms (probably Iraq, from what you wrote), give your brother some land, and then ensure that every title you want to transfer is considered de jure part of Anatolia.
The easiest way to do that last part is:
- (Save, and make a backup, in case you do something wrong!)
- (Don't forget to destroy Iraq)
- Give your younger brother some land in Anatolia, then a duchy that includes that land
- His primary title is now a duchy in de jure Anatolia
- Revoke all duchy titles in Iraq, and give them to your brother
- If you have time, it's preferable to manufacture reasons to revoke the titles without tyranny. Otherwise, tyranny!
- He is still primarily a Duke in Anatolia, but he is also overlord of every county in Iraq via all those other duchy titles.
- Check the de jure structure of Anatolia
- Left-click on the shield for Anatolia, ensure the "de jure" checkbox is ticked, ensure that the flashing yellow border covers the entire area you intend to give to your brother.
- If it doesn't, there may be some other cases of split inheritance that you'll have to resolve. Unless you have a LOT of time on your hands, your only choices are tyrannical revocation or doing nothing.
- Give your brother the kingdom of Anatolia, and make sure to tick the box to "grant lower titles".
- Check again that you've partitioned the realm correctly.
(If any of the checks fail, revert back to the original backup and try again. These complicated realm partitions often take 3-4 attempts to get right, so I don't think you should feel bad about this type of savescumming.)
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