They attacked each other and then retreated together to swing their swords at each other again. Although the weapons contract between humans and companions meant they couldn't hurt each other, it apparently did in certain situations. When a person used the full power of their weapon and attacked their partner, it automatically disappeared when it touched the partner's body. Or a unique shield was created so that it couldn't hurt them.
Essentially, there would be no scratches, shots, arrows, or anything else hitting them. That allowed Carl to last a long time in a duel with Miku. It was added with protection on his body. He was sure there would be no injuries even though they dueled for long.
Miku spun around with her swords. It was a paired sword, so it gave a whole spin reaching her opponent. It forced him to back away from her again. And he took advantage of it by throwing his knives at her upper body.
