Install iOS 12.1.2 for Dual-Sim Support
The iOS 12.1 update brought support for dual-sims on the iPhone XS and iPhone XS Max. iOS 12.1.1 brought Dual SIM with eSIM for additional carriers. If you skipped those updates, and want support, you'll need to download iOS 12.1.2.
With iOS 12.1/iOS 12.1/iOS 12.1.2 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.