It's normal behavior. They usually settle down some after they start to spawn. Keep an eye on them anyway, sometimes one or the other will get a bit too aggressive, tearing fins & such. The lip locking is fine. If they are the only angel-looking fish in the tank that is good. Any other angel, or fish they think looks like an angel will get chased around once they start to spawn.
My oldest pair will do a little lip lock thing every day when I turn the light on. After a few minutes they quit, and go back to hanging out together. My marble pair turn into terrors when it's spawning time, herding all the other fish into one far corner of a 65 gallon tank. My black pair needed to be split up with a divider after being moved to a 20 tall, the female decided to tear into the male, chewing off nearly half of the tail & dorsal. I packed that tank with fake plants, now the male has a few hiding places and they get along fine.
All pairs are different, just keep an eye on them, they should do fine. It's nice to hear they paired up so quickly. I've got 5 koi that I've been growing out, been waiting for them to pair up for the last couple of months. Seems they never cooperate when you want them to, and when you least expect it, especially when tank space is tight, they do what you've been waiting for.
Tolak