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."
At the highest level, Google Fi is a phone carrier operated by Google. In the US, it gives you data service on three mobile networks (T-Mobile, Sprint and U.S. Cellular), which your phone will intelligently switch between — it also uses Wi-Fi to make calls and send texts whenever available.
Getting this feature to work on a smartphone is a bit of a hack, and for now it will only work with the freshly released Pixel 4. Google is using Dual Sim Dual Standby (DSDS) hardware to connect to two networks at once, which isn't that crazy of an idea, but it's using DSDS to connect twice to the same network, that network being Google Fi. You'll have to have Fi activated on the internal eSim chip and have a physical card installed in the device, allowing your two SIMs to each pick one of Fi's MVNO networks. If you've been a purely eSIM Google Fi user, which normally needs no physical SIM chip, you'll need to order a physical SIM card, which you can do for free through the Google Fi app.
As the name suggests, "Dual Sim Dual Standby" only allows you to listen to two networks at once for incoming messages; you can't actually use both networks at once. If Google really wanted to flex on other mobile networks, it could make a Dual Sim Dual Active (DSDA) phone that works on Google Fi, which would give it the ability to connect and suck down data from two networks at once. Then imagine aggregating both of the connections into one super-speedy connection made of two LTE networks. It's not a totally unheard of idea—Samsung has shipped a "download booster" on its phones for some time now, which uses your WI-Fi and LTE connections simultaneously to speed up some downloads.
Doing this with two LTE networks would be more complicated, as DSDA phones require dual receivers, and the carriers might not like it. Google Fi already requires phones to be specially designed for Fi for its network switching (normal phones still work, though), and if you're doing custom phone hardware for Fi, at this point you might as well go for it.
Google says dual connect will be ready for the Pixel 4 as usual as part of a slow rollout that will take weeks to hit everyone. Google says it will "expand dual connect technology to more Designed for Fi devices" at some point in the future. A good bet for future support is the Pixel 3a, which already supports DSDS, just not on Google Fi. Another possibility is the Pixel 3, which supported DSDS during the Android 10 Beta, but the feature was cut for launch.