https://arstechnica.com/gadgets/2019/10/google-fi-will-soon-connect-you-to-two-lte-networks-at-once/

Google Fi is getting an upgrade today with what Google is calling "Dual Connect" technology—the ability to connect to two of Google Fi's licensed mobile networks at once for faster switching.

With Google Fi, Google is operating as a Mobile Virtual Network Operator (MVNO)—a company that doesn't build its own networks but instead resells network access owned by one of the big carriers. Instead of doing this for one network, Google does it for three. Google Fi gives you access to Sprint, T-Mobile, and US Cellular, picking the fastest network available at any given time. Normally, switching between these networks requires a small amount of disconnect time, but with this new "dual connect" technology, Fi phones will be able to hop between two networks seamlessly. Google says: "if you’re watching a video and Fi switches you to a better network, you won't experience any delays or pauses—you won’t even notice."