To be safe, do a backup of each device first. This support article outlines the various ways you can back up your iPhone, iPad, and iPod touch --> How to back up your iPhone, iPad, and iPod touch - Apple Support
Make sure you do this to each phone before transferring data:
What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support
That will ensure you really become the new owner of each phone and there is no residual data.
Read the support article Transfer data from your previous iOS or iPadOS device to your new iPhone or iPad - Apple Support to learn how to transfer data from your previous iOS device to your new iPhone, iPad, or iPod touch. Select one of the three options: Quickstart, iCloud, or iTunes/Finder. Quickstart may not work in your situation since at one stage both devices will have identical data with no data left from the other account. Quickstart is usually done where you have a brand new, empty device.