Apple is rolling out iOS 12.1 today, and if you have an iPhone Xs, Xs Max, or XR, you can use Apple's new dual-SIM feature... as long as you're not using Verizon on your digital SIM or as your secondary number. Both T-Mobile and Verizon have stated that their networks are not ready to fully support the dual-SIM tech but are working on getting their networks up to scratch "in the near future."
https://www.notebookcheck.net/Verizon-and-T-Mobile-don-t-support-iOS-12-s-dual-SIM-feature.354141.0.html