You have been friends, maybe even close friends for years - you, the husband and the wife. Now they're splitting up, and you're caught in the middle - how can you be a friend to both of them without ending up being the bad guy yourself?
Tell them both that you will not choose sides. Be honest with both of them, letting them know that you will accept all calls, emails, etc. from their ex, and will do your best to remain neutral. If there's a problem with that, you'll have to decide then, but don't pretend you're not in contact with the other spouse. Instead, it's best to be honest, but say, "I'm not taking sides. I'm Switzerland." This way neither will be surprised to find out you've heard from the other.
If you do find yourself siding with one, limit contact with the other. Once you've heard both sides of the story (don't pre-judge!), if you realize you are more sympathetic to one (for example, adultery was involved, and you are siding with the spouse who didn't cheat), you will have to decide if you care to continue your friendship with the ex. Clearly, you will at least need to limit your contact to bare minimums to prevent yourself from lecturing or scolding the offender.
Reveal no opinion on who is right or wrong. Unless you know for a fact that something is factual, do not take a position. Each person will have his or her own filter and will be playing back every incident through that filter. He said/she said is never accurate, both perceive the same incidents in completely different ways. Simply listen, be a sounding board, and "reflect" each spouse's feelings back to them.
Example: Him: "I just can't believe she'd do this to me, I mean, do you think she is having an affair? How could she?"
You: "I don't know, man. I can hear how upset you are, though. It's hard to believe it could go so wrong between you, isn't it?"
In other words, don't buy into his wild suppositions, despair, or anger. Simply be there for him.
Expect anything you say to one to get back to the other. There's rarely a mutual and simultaneous parting of the ways. One spouse decides to leave the other. Sometimes it will seem sudden and without warning, but more likely, if the spouse being left is honest, the signs have been there - s/he's just been denying it. Your first compulsion will be to commiserate, and a lot of times, this takes the form of you assuring (for example) the sobbing partner that you can well believe her husband's acting this way - you've seen it with past girlfriends, or whatever. In her frustration, rage, and fear, she will use this validation of her very raw feelings to prove some point to her formerly dearly beloved, and eventually, you will suddenly find yourself on the pointy end of a finger as he accuses you of betraying his secrets.
Resist answering emails from either of them. Although emails are quick and convenient, they're also impersonal and non-retractable. And they are in print. You don't want to put things in writing to either one of your friends right now, you just want to be a good friend. They can't see your sympathetic face, and they also can't put a tone of voice to it. They're going through a very tough time right now, and their nerves are on edge, rubbed raw with emotion - any little joke you make or bit of irony you add in an email is likely to rub them the wrong way, so don't compound an already tough problem by doing this. If you get an email, IM or text from them, call them back. All they need is to hear your voice. It'll help.
If all else fails, detach from both for the duration. This is a last resort, to be taken only if you find yourself being taken to task a second or third time for "betraying a confidence," etc. Remembering the previous step, you must know that each of them is feeling pretty much like they've been chained to a pickup hitch and dragged for awhile. Each of them is looking for any little thing they could seize upon to use as a scapegoat - something, anything, to blame for their present difficulties. If you find you present too tempting a target to them, make a polite exit. Tell them both, perhaps via email with each of them in the TO: line, that you love and value the friendship of each of them, but find yourself too far in the middle of things for your comfort. While you want to remain friends with both of them, you are no longer able to continue at the moment, that you want to be sure you do not intrude upon their private hell. Let them know that when it's all over, you'll be there for them both, and that they know how to reach you, but that for the time being, you want to give them the time and space they need to work out their issues once and for all - without you. Even if they try to get you involved, resist temptation.
Listen. Anyone going through a divorce needs someone to listen and not judge. This involves silence when you may not want to be silent. This involves keeping the information you hear confidential even when you are asked by someone what the person said. This involves paying attention when you may have heard the same story more than once.
Pray. Pray for both parties. It will be difficult for them to make it safely to the other side in one piece. Let them know you are praying for them, if they and you are religious.
Don't fight back if you find yourself on the raw edge of a baffling attack. Your friends are apt to lash out as a way of releasing the pent-up rage and frustration they feel. Breakups are messy, and divorces is even worse. If one of them flies off the handle at you, try not to react, and definitely don't fight back. Just take the tirade as best you can, and when a quiet moment finally happens, say, "Are you done?" Assuming s/he is, say, "Okay. That was horrible and I don't think I deserved it. I'm going to put it down to you're feeling awful today, and go home. I'm not down for being treated that way, but I love you. Talk to me when you're feeling a little more human-like." This is non-accusatory, will hopefully have the effect of making them realize they did a crappy thing by yelling at you when you aren't their problem, you're part of their support network, and also hopefully leaves the door open for them to return - but clearly informs them that you won't be sticking around to be their punching bags, no matter how much they need one. Being a good friend doesn't mean being a doormat, even if you're not going to throw a return punch.
Don't involve yourself to the exclusion of your own life. Don't you end up in a breakup situation because you feel so obligated to help your poor friends that you neglect your own relationship. Likewise, don't endanger your job by taking calls from weeping pals while on the company dime - cut them short. Your friends have made their bed. And so have you, and it's a fair bet that your bed's a lot nicer to lie in right now than theirs is - make sure it stays that way. Go home.
Sending each of them a card every week or so can really let them know you are thinking about them, and lift their spirits. Even if this is the only interaction you feel safe having right now, it can really help them both.
Consider offering to take their children to the movies or another fun activity. It's a hard and bewildering time for them as well.
