Unlike a traditional VPN setup that uses a single VPN server at a time to encrypt and transfer your data, a double VPN simply uses two VPNs simultaneously. With a double VPN setup, your data get transferred through two different secure VPN tunnels and gets encrypted twice.
For instance, the first VPN tunnel that you get connected to encrypts your data with 256-bit encryption and transfers it to the second VPN tunnel which then encrypts it again before transferring it. The technique essentially adds double security and privacy.
Although there are different types of double VPNs, it all works more or less the same and basically helps mask your IP address twice for added security. Typically, only popular VPN providers offer a double VPN feature, however, you can achieve the same level of double encryption by using two different VPNs and virtual machine setup.
You can install a free Virtualization tool like Virtual Box on your computer and run one VPN app on your computer and other VPN apps on the virtual machine. Although double VPN works great and offers more security, it can slow down your network speeds quite significantly due to added encryption and tunneling.