This is a very good question, and I do have some thoughts I can share with you. But my views on this take a more functional approach to getting the couple into therapy rather than the usual path of helping a couple work together with a therapist for the goal of improving the relationship and staying together. Let me explain.
From your description they are not good or easy candidates for couples work because of their resistance and lack of motivation for healing. In these instances, since it looks like they are heading toward a divorce, I appeal to couples to save themselves a fortune and make one of three recommendations. All of these tend to move them out of the boxing ring.
I recommend a divorce mediator (NOT divorce lawyers). A divorce mediator is someone specifically trained in putting together a memorandum of understanding that saves the couple tens of thousands of dollars in the divorce process. But what it more often than not does is let the couple stand on the rake of reality with a third party present. The divorce mediators are trained to detect how ready a couple is to move forward in a divorce and are usually in the position of being able to recommend a professional couples counselor to help with the emotional issues.
Secondly, when couples are this cantankerous I recommend they do a couples weekend where they use an intensive approach such as Imago, to get the issues out and clarified. In these instances I make this recommendation by saying they have spent X amount of time together and they should give it at least one weekend to see if there is anything left that can be salvaged. I also point out that they will want to understand what went wrong in this marriage because it is likely they will make the same mistake again with their next partner and they should try to learn from this relationship how to avoid making that error again.
Finally, I implore them to go for a one-shot assessment with a couples expert who would see them once, usually for an extended session, to help them with the next step. Again, the approach is a functional one as I say that a divorce is expensive and a mediator, a weekend, or an assessment can save a lot. This often appeals to couples at a level they intuitively know to be true. The message is: just because you are angry at each other doesn’t mean you have to both waste your resources on fighting through lawyers. This, somehow, has been an effective approach.
Wishing you patience and peace,