One big suggestion: tell them to seek out a local parents of multiples group. I know they haven't had the twins yet, but sometimes it's better to surround yourself with experts, pros, veterans, and other newbies when you embark on a big adventure like having twins. A good friend of mine had twins for her first birth, and she was completely overwhelmed trying to do it alone. But after she got involved in a multiples group, she had so many people who knew what she was going through because they had been there or were dealing with it themselves. It makes tough times so much easier when you know you aren't the only person who's ever faced fill-in-the-blank issue. On top of that, a good multiples group will have things like clothes and equipment swaps...with the big plus that everything comes in sets of two or more...
As far as the marital issues go, obviously, you can't solve their problems for them. They have to want to work on them, and they have to address it together. Otherwise nothing will be solved, and yes, the twins and every other unexpected twist in life will bog them down. The best thing you can do is continually point them towards open and honest communication with each other. Any time one of them starts to vent about the other, just ask them (nicely, of course, not judgmentally) if the spouse is aware of whatever the complaint is. It's amazing to me how many people DON'T talk directly to the person they are upset with but instead talk to friends and family or strangers about what's eating at them. Without knowing more about the types of problems they're dealing with, I don't know what else to say, except to avoid being in the middle of any problems; avoid taking sides; and do what you can to be a support to your friends in this difficult time. Best wishes to all of you.