Apple watch Cellular Standalone

It needs to be paired with a phone. It has limited functionality if the phone is not present.

How to use your Apple Watch without your iPhone nearby - "If you don't have your iPhone with you, learn what you can still do with your Apple Watch."https://support.apple.com/HT205547

The Apple Watch shares the iPhone’s phone number. It never gets its own.

The Watch will need to be paired with an iPhone but that iPhone doesn't have to be nearby all the time. If you have two iPhones you could do what you seem to want to do.