Wow, interesting. I've been reading up some on them b/c I want to add some, since right now I only have corydoras. I need some better algae control.
Do you only have the 2, or are there more, but you just noticed those 2 behaving that way today? I've read that you can't really keep 2 together, b/c the stronger one will pick on the smaller/weaker one, even if male & female. But especially if they're both male, they'll definitely fight over territory.
It's difficult to see your photos, but are they fully grown, or is the smaller one younger? If they're the same age, I'd say the smaller one is a female. In the 3rd & last photos, it appears that the larger one has more coloring & patterning, which would also be consistent with it being a male & the other one female. I do not believe that looks like mating behavior if so. It appears that the larger one is butting the other one, & that may not be lethal, but it would be stressful for the smaller one.
If you only have the 2, & have enough room for 1 more, it might be most helpful to add another female.