同床异梦 – tong2chuang2yi4meng4
Literal meaning of each character = same – bed – different – dream
This is a very easy idiom to understand and use. It basically describes a marriage where the husband and wife pretty much do their own thing. Quite likely there is no romance or love left in the marriage and the staying together is likely only for convenience sakes. The only thing they share is a bed, whereas their dreams and goals are different.
Perhaps they stay together for the sake of the children, but in any event emotionally the marriage is pretty much over and they both know it and likely everyone else knows it too.