Nodding, smiling, staying friendly etc could all be seen, by either or both of them, as lack of support.
You need to tell each of them that you can't possibly be unbiased, you're finding the whole situation upsetting and that they need to talk it out with someone else, possibly a counselor.
That is especially the case if either of them is hoping the relationship can be saved.
Is it me or am I getting mixed messages
Is anyone interested in the terror attack in Moscow?