Home of the LEDmePlay. Since 2013.
LEDmePlay X-Link is a hardware extension which allows to connect two LEDmePlays via a cross cable in order to play games against each other. Technically, it is an ansynchronous serial connection using the UART (=Universal Asynchronous Receiver Transmitter) protocol. It uses the additional Serial1 interface of the Arduino Mega to avoid interferences with the standard serial connection used if the LEDmePlay is connected with a computer. Currently, the racing game Grand Tour makes use of LEDmePlay X-Link.
It is easy to a add a link interface to an existing LEDmePlay (of course it is easier if you directly plan to integrate it). You need a 3.5mm stereo jack plug which needs to be built into the housing of the LEDmePlay (typically, on the rear side beside the other connectors). Furthermore, you need a 1KOhm resistor that you have to connect to the RX1 pin (pin 19). After that, you can connect the 3.5mm plug in the following way:
Tip 1KOhm resistor connected to RX1 (digital 19)
Ring TX1 (digital 18)
With regard to the stereo jack plug you need a cable with 3.5mm stereo jacks to connect two LEDmePlays. However, it is necessary that the cable is a cross cable so that the TX1 pin of one LEDmePlay is connected to the RX1 pin of the other LEDmePlay and vice versa. The grounds need to be connected directly. Since it is not possible to buy such a cable, you have to solder it yourself. A good choice is to push the three lines through a heat shrink tube to let the final cable look a bit more professional (it is also more robust).