Can I connect 2 Apple watches to my cellular plan
Can I connect 2 Apple watches to my phone with a cellular plan
Apple Watch, watchOS 4
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Yes, But...
You can only use 1 Apple Watch at a time
https://support.apple.com/en-us/HT205792
The Family plan (if supported by your carrier) can allow a parent (or elderly care giver) the ability to pair and manage additional Apple Watches (kids, aging parents, etc...)
But each Family member Apple Watch gets its own phone number, and you get charged whatever the cellular providers charges for an extra line in their Family plan.