Install iOS 12.1.1 for Dual-Sim Support
The iOS 12.1 update finally brought support for dual-sims on the iPhone XS and iPhone XS Max. iOS 12.1.1 brings Dual SIM with eSIM for additional carriers.
With iOS 12.1/iOS 12.1 installed, you'll be able to add in a second phone line on the iPhone XS or iPhone XS Max without swapping SIMs.
You'll need to wait for your carrier to support the feature, but many carriers inside and outside of the United States have added the feature with the release of iOS 12.1.1.
For more information, check out Apple's website.