It is important to be cautious when combining different medications, especially when they contain the same active ingredient. In this case, since you have already taken two Tylenol Extra Strength tablets, which contain acetaminophen, it is essential to check if the Nyquil product you want to take also contains acetaminophen. Make sure you do not exceed the recommended daily dosage of acetaminophen to avoid potential harm to your liver. Take the time to read the list of active ingredients on the Nyquil packaging before taking it.