Dual SIM iPhone instructions
https://support.apple.com/en-us/HT209044
Dual SIM Apple Watch instructions
https://support.apple.com/en-us/HT209043
Basically starting with the iPhone XS and newer dual SIM support was added.
- XS
- XS MAX
- XR
- 11
- 11 Pro
- 11 Pro MAX
NOTE: You will then have to pay for a full cellular plan, and your carrier that supports the Apple Watch MUST ALSO support eSIMs.
OR your company's carrier must support eSIM, and your personal carrier just needs to support the Apple Watch
iPhone Cellular Carriers that support the Apple Watch, which models, the radio frequencies each model uses, and the country where they offer service:
https://www.apple.com/watch/cellular
You will have to check with the carrier to see if they support the iPhone eSIM.