Primogenitor True Form.
The two of them aren't speaking with their mouths. They are communicating with their minds so the onlookers can't hear what they say to each other. To them, it just looks like the monarchs are levitating in the air and grinning at each other. It is as if they have an inside joke that only the two of them know about. They didn't know of the violence that is currently brewing within them, waiting for an outlet.
But the grinning didn't stop. It became weird and unnatural but it didn't stop. Soon the onlookers had more pressing matters to deal with. They felt the two Monarchs release the pressure of their existence in every direction.
It is as if the monarchs want to crush each other with their pressure. The onlookers paled immediately because of it. It is not a good sign at all. It is as they say, when two elephants fight, it is the grass that suffers. They are the grass here.