Share Connectivity E-Series

I am using the E-Series evaluation board for a project, and I would like to know (if anyone could manage it) how to share 3G connectivity with some other board (the one I am using is the esp8266).
The idea is to connect both boards so that they can connect to other equipment developed by the company where I work.

You can connect another MCU to the E Series by a number of interfaces. The most popular are serial (UART), I2C, and SPI. From that point, you’d need to write custom code on both sides to exchange data.

It’s important to note that Particle cellular devices are not routers! This is especially true of Gen 2 devices (Electron/E Series). The cellular connection is slow, and a limited number of socket connections are available (7 on Gen 2), and the data allowance is designed for IoT, not general networking, This means that the Particle devices are not a good way to provide backup Internet to a computer or LAN, for example. And, generally speaking, not a Raspberry Pi, either, though that can be made to work in limited circumstances.

You could connect an ESP8266 by UART and exchange data, but there is no built-in facility to route IP between the Wireless LAN and cellular. The only practical thing to do is write an application-level gateway, and that still would require significant code on both sides.

Is there any library or functions, of Particle, already written to exchange data between boards through the UART?

The serial tutorial has an example of communicating with an Arduino over serial however any solution you come up with will require significant code both on the E Series and the ESP8266.