Is it normal; both cheating on each other, still together?

I think you should stay out of it.

It sucks to be you right now.

If you're concerned about the kids and how they may be negatively affected (because depending on how old they are, they may have some kind of "vibe" that something is not right, they are very intuitive) maybe you can sit each of them down separately, alone, and talk to them about that, and just about that person's infidelity, not mentioning the other person, even if they remark on it. Have them concentrate on how the kids can or will be affected. Chances are deep down they are thinking about this as well, which may be part of the reason they are pretending they don't know about their cheating on one another. Once they realize by your speaking to them that others have noticed and are concerned about the kids (obviously something you share with both parents, which is very fortunate for the kids that all their family is there and wants the best for them), they will think about their situation(s) even more, and perhaps decide to change it so that the entire dynamic and environment is one of trust, love, understanding and cooperation.

The fact that they are together and continue to peacefully and functionally coexist says a lot for their potential to resolve this in a functional manner. Make sure you let them know all this, and make sure they know anything said between you stays between you and that one person. Most importantly, let them know you are not trying to butt into their relationship or tell them how to live their lives because if it were only them it wouldn't affect you as much since adults make decisions themselves and live with the results themselves. But the children had no choice in the dynamics of their environment. They already know this behavior must stop and they have to grow up sometime. If you feel compelled to say something, be sure to frame it without blame or a moral punishment stick to beat them with, just with concern for what they kids are observing, and how it is shaping their lives, whether or not the adults think they're playing it off well enough that the kids can't tell something isn't quite right.

As long as they're fine with it, who are you to worry about it? I've heard something similar before, a relationship where they allow each other to cheat. They're not even ashamed of it, they just tell each other they've slept with someone else and take that from each other without even raising an eyebrow. I think they're the happiest couple of people I know. Nothing wrong with that.

I agree with bird, as fucked up as it is its really none of your business to get in the middle of it. It might even backfire and your relationship with them would get bad.