It's a little more peaceful. They started up again a couple of days ago, and I (very bad DBing here) totally lost my temper, told them they were both as bad as each other and I wasn't going to be involved in their disputes anymore. I said the way Eldest spoke to H was unacceptable, and it was no wonder H wasn't particularly warm towards him or inclined to cut him much slack given how reactive and disrespectful he was. And I said H was goady and nit-picking and controlling and it was no wonder Eldest was reactive and surly with him. And as neither of them were interested in making any changes to how they acted towards each other, they could work it out between themselves but I was no longer willing to listen to either complain about the other, no matter what.
I think I have been trying, more or less, to do 'united front' parenting with H. And that works up to a point - but he is such a whining control freak when it comes to Eldest. And Eldest is a young man: the strategies that you might use with a younger child just don't apply to him. This is more of a relationship issue that a parenting issue. And both of them use me to triangulate: Eldest will complain about his father (and I validate) then H will be annoyed at me because I've been validating Eldest, or H will whine and moan about Eldest, and I validate, but if I do anything other than accept full blame and responsibility and vow to do better, he's displeased, and I am just so so so sick of it. A few days ago it got to the point where I'd left the room to do some work in my home office, and my phone was exploding with them both text messaging me complaining about the other. Lockdown has of course amplified this, but I am so done.
It was, as you can imagine, quite a lecture. Both of them were shocked. But I don't care and I mean to stick to it. Eldest isn't in any danger from H - H can be unpleasant and mean and quite cutting, but Eldest will have to learn that's the reaction he gets when he's so disrespectful and reactive. And H will have to learn that if he wants a better relationship with Eldest, he will have to look at his own approach. And until they learn to do that (both of them - and I hold out no particular hope) nothing will change. But I won't participate any more.
Both have tried testing these boundaries. Eldest tried, and I just gave him a warning and he left the room and was fine. H wanted to complain about how unfair it was that I said they were both as bad as each other. I said 'Yes. I appreciate you don't like my opinion. I don't plan on sharing it with you again but it is my opinion. And I don't plan having this conversation with you either now or in the future,' and I went back to my book. Unusually, he accepted that in fairly good humour.
So we will see how this new approach goes. I have noticed that the more assertive I am, the better for me things tend to be.