Another example is you can have up to 10 touchID fingerprints, but only 2 faceID's. So in reality you have 1 standard and 1 with sunglasses, but then your partner can't unlock the phone quickly to take pictures or make a call.
You don't have two set up Face ID twice for the same person with/without sunglasses. I unlock my iPhone XS with prescription sunglasses on all the time and I only have set up Face ID once with one face. Some sunglasses aren't compatible with Face ID.