If you want to add another Verizon line to a dual SIM phone, you would purchase another line/service plan and then a second SIM is installed to the phone. You then pick between which number you want to use on the phone as it's like having 2 phones in one device. Some apps won't separate the 2 lines though so things can be like a catch-all bucket with those apps.
If the phone is locked to Verizon, you can't get a different carrier's SIM to work on the phone until the phone is unlocked. Then you can run a Verizon SIM and some other carrier's SIM on the phone.
Here is a helpful link with more information on Dual SIM that explains the details as well as steps on how to setup, and Troubleshoot the service. https://www.verizon.com/support/dual-sim-with-esim-faqs/
-Alex
