Jack wanted both Doctors on his side, he didn't want to fight with either one. But as Ten seemed content to keep speaking about Ianto in such a manner, Jack had little to no choice about how he reacted. Really, Ten, you should have known better than to besmirch Ianto's honour about four hours after Jack watched him murdered.
"Yes, we know," Jack replied. "However, do remember that you knew she was still alive," the last word was hissed, Jack's eyes narrowing as he looked at Ten. "I lived for years thinking they all had died not that you had managed to spirit them off to a parallel world. So do not make yourself out to be the martyr here." Jack swallowed hard, his eyes flashing with anger. "Yes, Ianto was a part of it. But in the side that was turned into cybermen trying to fight back. Remember that point."
Almost growling, Jack turned away from Ten and approached Nine, unable to say anything. He knew Nine was on his side, and taking those steps away from Ten felt like he was walking away from the other man at long last.
Ianto would have said it was about time, but the thought was only bitter in Jack's mind. He said nothing when he stopped beside the other Time Lord -- just stood there looking to be the broken man that he was.