How can I use same Apple ID on my iPhone 15 Pro Max and 16 Pro Max?

Simplest Way to Use the Same Apple ID on Two iPhones

🟦 1. Log in on the Second iPhone

• Open Settings → Tap "Sign in to your iPhone" at the top
• Enter the exact same Apple ID and password as your first iPhone
• If prompted "This Apple ID is already used on another iPhone", simply select "Merge"

🟦 2. Disable Potential Conflicts

• Turn off iMessage sync (to avoid message confusion):
  • Settings → Messages → Turn off "iMessage" (on one iPhone only)
• Disable FaceTime sync:
  • Settings → FaceTime → Turn off (on the same iPhone)
• Prevent automatic app downloads:
  • Settings → App Store → Turn off "App Downloads" and "Updates"

🟦 3. Done!

Now both iPhones can use the same Apple ID simultaneously without interference.

In case this iPhone had a different Apple Account, did you remove it from "Find my" and reset the iPhone in question to factory settings?

Restore your iPhone, iPad, or iPod to factory settings using a computer - Apple Support

After that set it up again from a backup of your other iPhone or as a new device, not using any backup data.

Set up your iPhone or iPad - Apple Support

Restoring from a backup is one option you have. If you want to start fresh, set it up as a new device and add needed apps back later.

Setting up the same mail accounts should also be no problem, despite the fact, that read mails will be marked as read on the other device after first opening them on the first one.