Apple released iOS 12.1 yesterday and supports the iPhone XS and XR by using a digital eSIM for a second SIM card. Although Apple has additional technical support for eSIM cards, none of the major US carriers are ready for it. According to a report by MacRumors AT & T, Verizon and T-Mobile are delaying their adoption of eSIM by the end of this year. (Sprint was not included in Apple's original announcement, however, telling PCMag that it will someday support eSIM.)
So, if you're a US cell phone customer, you can not really use it a second SIM card with your phone, unless you use one of the international carriers that supports eSIM.