hey longer, quick question, maybe two......are you absolutely sure you've got a male and female AND do you have other angels in the tank with them?
reason being, I've had 3 angels before and had 2 display this same behavior.......ultimately, I had to remove one of them because while the lip locking went on, things apparently got worse. My one angel suffered a good beating while I was at work one day and ultimately died after I had removed him to a hospital tank to treat him for his injuries. Evidently, they had been fighting over the female in the tank and the tougher guy won....... so if there are more than the two you speak about, watch them closely so yours might not suffer the same fate as mine did.
EDIT: whoops, just read your other post in regards to getting a breeding pair (the only inhabitants of a 90 gallon). I guess you can just look to this for future references but still keep an eye on them for this behavior. My breeding pair never lip locked once.