First of all, Ahli did not even realize that Dragon's Breath had joined the second persona's hand. He must have been too preoccupied with his own plan to deliberately lose. And by then Ahli knew how he had been an ass, putting himself above other, in this case his other self. And it would be understandable why all the wrath came from.
DRAGON'S BREATH
[Magic Spell]
Dragon's Breath deals 4 damage to any target.
Mana: -
As for the second matter, it is possible to target oneself using Dragon's Breath, but the second persona? The display of pride that he showed was something that was actually a call back to a time before all of this happening, a time when a casual play was better ended with one's hand.