To tie multiple Arduinos together you need to come up with some kind of networking topology and some form of networking protocol.
- The Arduinos must not (electrically) interfere with each other
- There must be some way to know which Arduino is sending what and where to
The two points could be addressed by having a separate connection for each Arduino but that ends up complicated, and there are only limited resources on an Arduino for creating such connections (you really need to use the hardware UART ports and the Uno has only 1. The Mega has 4).
A better solution is to use a bus system (also known as a multi-drop network). There are a number of existing and well supported methods for doing this. In the automotive system the CAN Bus is used. In industry the RS-485 system is heavily used.
Personally I prefer RS-485 since it is well established. There are a number of standard protocols that run on RS-485, such as MODBUS, but for the Arduino world I wrote my own light weight protocol specially which is far simpler to get going with. You can find out more here: https://github.com/MajenkoLibraries/ICSC