There are two types of SIM cards in use today: V-1, or pre-2001 SIM cards, and V-2, or post-2001 SIM cards. You can use a SIM-card duplicator to make a copy of a V-1 SIM card and have two phones that have the same SIM card and phone number. However, it is not possible to clone or copy V-2 model SIM cards because companies have incorporated anti-cloning and anti-tampering features that will prevent a copy from working. So as long as your current SIM card was purchased before 2001, you can have a dual SIM card for the same number.
Locate the International Mobile Subscriber Identifier (IMSI) number on the SIM card. The IMSI provides a unique identifier to each phone on the network. In the United States, it's a 15-digit number. Write down the number for your reference.
Insert the SIM card into the SIM-card duplicator device. Follow the instructions given with the device, entering the IMSI when prompted. Extract the SIM card when instructed.
Insert the second SIM card and follow the instructions included with your SIM-duplicator device to copy the original SIM card data to your new SIM card.
Insert each SIM card into a separate phone. Note that, depending on your network, you may need to turn one phone off in order to use the other one.