Chances are that you will feel comfortable speaking to only one parent and not the other, and that’s okay. It could also be that even the parent you thought might be receptive to your words just isn’t, and that’s okay too. While you may be speaking very respectfully and gently, it’s possible that for the same reason your parents were not able to accept uncomfortable things from each other, they may not be able to accept hearing such things from their children.
So, if you see that what you’re saying isn’t being taken well, stop. It’s very likely that you’re not the right person to be saying it.
And even if you think that your words are falling on somewhat fertile ground, don’t say too much. Often you accomplish most by leaving them to think about your ideas rather than pressing the point. In such cases you can just repeat how much you recognize that they are dealing with a very difficult situation in the best way they can, and stress that you don’t know any better than them how things should be done. You definitely don’t want to make it sound like you’re smarter or a better person than they are.