After an Apk teardown by XDA, It was discovered that Google is working on using dual SIMs to speed up network switching on, Google Fi.
GOOGLE FI DUAL LTE SUPPORT
The feature works by using Dual SIM Dual Standby, and Google Fi uses a Physical SIM and eSIM to connect to two networks at once. Only one network is in use at a time, but Dual Connect makes the switch between networks much faster and more seamless. You could be watching a video, and the network can switch without causing the video to buffer or stop. Both networks are always ready for action.
This update comes in after the launch of Pixel 4, which supports this dual SIM LTE feature of Google Fi. Google is putting both of those SIMs on the Fi network. That way, each can look for which one is strongest and switch to it on the fly without interrupting data. Pixel phones could switch between those networks before, but not quite this seamlessly.
This feature allows them to be on standby and not active to save battery life. Phones like Pixel 4 don’t have much battery to spare for such functions.
Doing this with two LTE networks would be more complicated, as Dual SIM Dual Standby phones require dual receivers, and the carriers might not like it. Google Fi already needs phones to be specially designed for Fi for its network switching.
Google is experimenting with lots of different features and plans for Fi. It had started by offering a clear per-GB plan, but it eventually gave up and added an Unlimited plan last month. It also provides data SIMs at no additional cost, a free Google-hosted VPN for Android users, and no extra roaming fees on international data.