How do I sync two iPads to each other, so they both contain all the data from each other? Thanks.

How do I sync two iPads to each other, so they both contain all the data from each other? Thanks.

I have an iPadAir2, iOS 10.0.2 which I set up from a backup of my iPad 3, iOS 8.4.1, via iTunes several weeks ago. Unfortunately I continued to use both iPads since that time, as I didn't yet have a case for my new one.

Now, I would like to make sure that all relevant data, including Notes, photos, downloaded PDFs in iBooks, calendar, etc. from both iPads is all on the iPad Air2, before I do factory reset on my iPad 3 to give to my husband.

Help, please! I understand how to backup the ipad3 & install this onto the Air2, but I don't know how to save the work I have done which is unique on the Air2, so I don't lose it If I install the most recent backup of the iPad 3. Hope this makes sense, thanks in advance.