Use Dual SIM iPhone with Apple Watch cellular models
If you set up multiple cellular plans using Dual SIM on your iPhone XS, iPhone XS Max, or iPhone XR, you can add multiple lines to your Apple Watch with cellular, then choose the one your watch uses when it connects to cellular networks.
Note: Each iPhone cellular plan must be provided by a supported carrier and must support Apple Watch cellular.
Set up multiple carrier plans
You can add one plan when you set up your watch for the first time. You can set up a second plan later in the Apple Watch app by following these steps:
Open the Apple Watch app on your iPhone.
Tap My Watch, then tap Cellular.
Tap Set Up Cellular or Add a New Plan, then follow the steps to choose the plan you want to add to your Apple Watch.
You can add multiple lines to your Apple Watch, but your Apple Watch can connect to only one line at a time.
Switch between plans
Open the Settings app
Use Dual SIM with Apple Watch Series 4 (GPS + Cellular) and Apple Watch Series 3 (GPS + Cellular) and iPhone User Guide.