How do I get my Arduino to reset another board?
MCU reset pins are normally active-low with pull-ups, which means you ground them to trigger the reset and leave them open to run normally. There's a special kind of output called open drain that's perfect for driving this sort of pin. Unfortunately, it seems like Arduinos don't support open drain outputs directly. This page suggests putting the pin in input mode to produce the open state and output mode to produce the grounded state. Alternately, if the reset is totally controlled by the Arduino (i.e. it never toggles itself), you can drive the pin directly with a normal output.

All this assumes that the MCU board is using the same 5V or 3.3V supply as the Arduino. If this is correct, it's safe to do what I've described. But if the MCU board uses a higher voltage than the Arduino, you'll need to use a transistor (NPN or NMOS) to pull down the reset line.

If the reset pin is active high, you'll have to drive it with a normal output, and the voltages probably have to be the same. You can figure out the polarity of the reset by measuring the voltage on the shorted pins with a multimeter. You can get the board voltage by measuring both pins separately.

You'll also need to connect the grounds of the two boards together to provide a return path for the current.