How can I connect two mutually exclusive output on one pin of a MCU?
Connect the output of the optocoupler to the same Schmitt trigger input as the button.  That way, the button and the open-collector output of the optocoupler will be ORed together.  The signal from the optocoupler would be debounced like a button (even though optocouplers don't bounce).

The 74HC14 doesn't have an open collector (or open drain output).  If you connect the output of the optocoupler directly to the μC pin, it will be in parallel with the output of the 74HC14.  The optocoupler would not be able to drive that node low against the output of the 74HC14.