I think you could make a statement on this pattern to both of them.
I very much sympathise with how the relationship with X has been difficult for you
Please consider that on a wider scale the community needs to rebuild and it's very important to avoid heat and tension as we do this together.
I am therefore appealing to you to avoid communicating with X because of the trouble there has been in the past
I am also asking the same of X.
I have done what I can to help but in the end it's bad chemistry between the two of you that cannot be helped by anyone else.
The best option is for you to use Ignore to cut out posts by X, see below. The alternative can only be heroic self restraint which is asking a lot of you.
Please be warned that for the sake of community peace if you find you simply cannot refrain from responding to X in sharp style, I will have to take drastic measures such as cancelling your ability to post a Personal Conversation at all, or placing your forum posts on moderation which delays their appearance on the board; or both.
I would strongly suggest you place X on Ignore so he no longer annoys you. You can do this by clicking his username anywhere you see it, then on the popup Membercard, on the right click Ignore.