Arduino NANOs sharing input signals and controlling Pixel Ring blowing up
You should add resistors on signal lines between the Arduino outputs and the LED boards. A few hundred ohms  (maybe 470 ohms to 1K) should suffice, if your speed is high you may have to use a lower value depending on cabling etc. 

The reason is that the ground on the LED boards will bounce around relative to the Arduino supplies as the LEDs switch. If it bounces outside the supply range by more than a few hundred millivolts, significant current will flow in the protection diodes on either chip. If that current is too high, it can cause latchup of the CMOS chip which will generally be destructive if there is a lot of power supply current available.