You cannot have two SIM cards associated with the same phone number at the same time. In order to switch between two SIM card you have to either call customer service or sign into your MyVerizon account online and manually activate the SIM card. This action deactivates the previously used SIM card. You have to repeat the process every time you want to use a different SIM card for your phone number.

It much simpler to swap the active SIM card between multiple devices to designate which phone is connected to the network at that time.