Angel Fish Behavior: crazy or mating?

I have two koi angel fish, both approx. 1 year old who grew up
together. Both are about 3" in the body, with substantially longer
fins. They have always bickered a lot, chased each other around the
tank, took turns being the bully. But lately their behavior has become
much stranger and intense. They spend a lot of time facing each other
and twitching, then locking lips and spinning around. Very weird. The
guy at my LFS told me that this is mating behavior and that this means
they are a pair. They are the only fish in the tank (with the exception
of a vampire pleco.) They have actually done the lip lock thing for a
long time at periodic intervals, but the twitching is new. They also
dart around the tank at breakneck speed and nip at each other quite a
bit, usually on the body (I've never noticed them nipping at each
other's fins.) Sometimes they rapidly flutter their fins and tail as
well. Their frenzy for food has also been lacking of late, and where
normally the frozen bloodworm gumball that's usually devoured in about
1.5 minutes for dinner, is only picked at over the span of an hour or
two. They have still enjoyed their frozen brine shrimp though, but I
don't think this is a good diet on its own.

I would like to hear from anyone who has had angels that acted
similarly to this, and what it means. I hope it isn't something I
should worry about; they have always been fantastically colored,
healthy, active and alert and I am quite attached to them so I want to
make sure I'm doing everything that I can to keep them that way.

TIA,
Jen