At that size and from what you've said it could definately be spawning behavior. Lip locking is normal in that case, and so is going to the bottom and sitting. You might also see one cleaning the nest site (they do this with their mouth, so if you see them picking up gravel or biting in a certain spot that could be what they're doing).
Even if they're spawning, keep in eye on them since they can still injure each other.
It may also just be normal bickering. The one might have taken over an area of the tank so it's staying in that area to keep the other one out.