You can add multiple devices to Apple Care One.
If you have the individual Apple Care+ plan, that's specific to one device only. You can replace Apple Care+ with Apple Care One and add the other devices for a fee.
click here ➜ Add or remove a device from your AppleCare One plan - Apple Support
Switch to the Apple Care One option on your covered iPhone first, by selecting it in Settings ➜ General ➜ Apple Care & Warranty. Once you have enrolled you can follow the steps in the link above to add more devices to it.
Switching to Apple Care One should automatically cancel your Apple Care+ option and offer a prorated refund for the remainder (of the current month if its monthly, or of the plan if it was a one time purchase)
Do not cancel Apple Care+ manually before enrolling in Apple Care One.