As an alternative to using the O/U markets to back another goal being scored, it is simpler to lay the current score. The relative odds should be equivalent in liquid markets. I have attached a trigger, where the correct score favourite is layed at 85 min because the current score will always be favourite at this point in the match. I have just tested this successfully and it placed the first bet on the Ross County v Celtic match when the score was 1-2 at odds 1.44. The bet won.