Apple's dual SIM functionality in iOS 12.1 currently has a negative effect on Verizon coverage. According to a report from PCMag, when a Verizon SIM is pushed into the "secondary" position in this dual SIM setup, the connection gets moved to Verizon's 2G CDMA coverage. Not only does Verizon's 2G network have less coverage than its LTE network, it's also much slower and has no MMS support.
When asked about the issue, Verizon explained that "your Verizon service will be degraded due to the current software configuration" if you actiavate another carrier's service on your iPhone's eSIM. Verizon went on to say that based on its talks with Apple, it believes that the issue should be resolved before the end of the year. Until the problem is fixed, Verizon won't activate its service on any eSIM, including its own customers' iPhones and iPhones on other networks.
Dual SIM functionality is handy for several reasons, like having your personal and work phone numbers on a single device. It's good to see the feature finally launching on iPhone, but this Verizon bug is a pretty significant one, so it's a good idea for Verizon to hold off on activating eSIM service until everything is ironed out. It is worth noting that the bug doesn't affect international roaming, so if you're traveling abroad you can push your Verizon SIM into the secondary spot, so long as you remember to make it your primary SIM when you get back to the U.S.