2 phones to 1

You can SYNC data to iCloud. This is completely unrelated to backing up to iCloud. If both old phones have the same iCloud ID/Apple ID you can sync them to each other, and to the new phone using iCloud.

Here is an explanation→What's the difference between device storage and iCloud storage? - Apple Support

iCloud+ has 2 independent functions. iCloud backups, and iCloud sync. It’s confusing because they both have the same name prefix. But they have no connection with each other. Go to Settings/[your name]/iCloud. You will see a bunch of switches (with iOS 16 or later also tap Show All). When you turn on a switch that data type will sync to iCloud. If you do this on multiple devices that share an Apple ID the selected data will sync to all of those devices. 

You can also turn on iCloud Backups, as you apparently have. This will back up your phone every night if the phone is plugged in, connected to Wi-Fi and locked.

But note that anything that you sync by the first method will be excluded from the iCloud backups.

Finally, some things do not sync device to device or device to iCloud, such as Contacts, Calendar events, Notes, and Reminders. These sync to your email account servers. So to transfer them just add the same email accounts to the old and new phones.

As we mentioned above, you cannot "merge" the data of both phones over to another third phone.

You can move data from the backups of only one of the two phones.....not both..... over to the "new third" phone.

If you try to do what you are thinking about......

You move the data from one of the two phones over to the third phone. Now the third phone will look like the first phone.

Now you move data from the other phone over to the third phone. That will overwrite what you have already done and the third phone will now look just like the second phone.

Are you asking about combining the data on both phones onto only one?

If yes, you can't.

If no, what is it that you are trying to accomplish?

XM42 wrote:

I’ve been reviewing posts on the subject, but haven’t been able to find a clear answer.

You haven’t stated what the subject or question is, so it isn’t possible to give a clear answer to a question that you haven’t posed.