It's possible to link two Polycades to play multiplayer games!

This can be achieved in a few different ways, depending on your goals.

Option #1: Software Link

If both Polycades have their own computer inside, you can use a free software called Parsec in order to link them. Parsec allows one computer to "login" to another, which even works over the internet, allowing you to use this to link your Polycade with your friends Polycade somewhere else in the world (note: the farther away the machines are from each other the higher the lag will be). 

In order to make it work, you'll install Parsec on both Polycades, and then set one as the "host" and the other will connect to the host. From there, your Polycades will behave as if you were both playing on the same system, allowing you to play retro or modern games multiplayer (even if the games don't support online play).

Key advantages: 

• Enables both Polycades to function as independent, two player multicades when desired, or as a single 4 player multicade when desired.
• Allows the two Polycades to be linked over the internet, or locally.

Disadvantages:

• Setting up the link when you're ready to play 4 players takes a little time, effort, and know-how.

Option #2: Master / Slave

It's possible to setup two Polycades as if they were a single, 4 player machine. In this scenario, you only need one computer - the 2nd Polycade doesn't need a computer. To achieve this, you will likely need the Polycades next to each other, as you'll run the controller USB cables and HDMI cable from the 2nd Polycade (the "slave") into the first Polycade (the "master"). The master machine will then think that it has two monitors connected (set the monitors to "mirror" each other), and 4 controllers, effectively behaving as a single 4 player machine across both Polycades. 

Key advantages: 

• Save money by only purchasing 1 computer to power both Polycades.
• Machines permanently behave as a single 4 player machine, no setup required when you're ready to play.

Disadvantages:

• Polycades can't be used as independent, 2 player machines.
• Both Polycades must be linked by physical wires, requiring them to be close enough to eachother.
• Potential confusion around player controller order, since one machine will be "player 1" and "player 2", and the other will be "player 3" and "player 4".

Additional options

1. Many Steam games have built-in online play.
2. Retro console games generally support "Netplay", allowing you to connect to other players online.