My Sister and I have decided that my parents relationship is what it is.  Somehow, as hard as it seems to us, they're used to it and there's nothing we can do.  In fact, in a weird way - it's all they know and they wouldn't know what to do without arguing. 

That said, we make a point of listening to our father (or our stepmother when she talks to us) and being as sympathetic and supportive.  If there's room in the conversation, we offer gentle advice when either appear to be open to it. 

My sister and I focus on taking our Dad OUT and showing him a good time as often as we can.  This usually involves a short walk, or a trip to Denny's - but it's great for him (and great for his wife to have some alone time too).  We also bring movies that we can all enjoy together.

It's a very touchy situation.   Good luck, and remember - you probably can't fix it, but you can be "there," for him and supportive.