The way Kamui practically flinches away him makes Duo cringe inside as well. He doesn't want Kamui worried about upsetting him or anything else. His mind runs, juding the risks and benefits. Kamui wears himself out when he heals, trying to do too much. Before, Duo was injured so badly, Kamui could have poured enough energy into Duo to kill seriously injure himself or even die. That'd hurt Kamui because he didn't understand. Now, Duo's only injured a little, the burn not even enough to take away his ability to speak. Misa was able to heal his lungs last time without dropping so if that's all Kamui does, he should be fine as well.

Coming to a choice he likes, Duo takes Kamui's hand in his own and pulls it down to touch his chest above his lungs. "This time, don't wear yourself out." He doesn't add that he won't ask Kamui to heal him again if his brother does heal until he passes out.