most PLC's I have encountered run at higher voltages than the arduino, so direct connection would make a wonderful smoke generator (ie 24 volt signal going into a 5 volt digital pin)
if it were a 5v setup since both sides are high impedance there should be no problem connecting them in parallel as long as your grounds are connected. your biggest issue is how to take a higher voltage system and drop it down to 5v without too much current loss, so that your PLC doesnt register the signal