Some friends of mine have done just that. They moved in together after being a serious couple for about three years. It wasn't feasible to live together before that point due to the young ages of their children (from previous relationships), work commitments in separate cities, financial issues etc. But as soon as they were in a position to do so, they rented a house together and moved in, with her two children. His older child was also there for about 40% of the time too and they all had good relationships with the co-parents of the children involved.
I think they realised quite quickly that actually, all living together wasn't really working for various reasons. They are both quite different personalities - she is quieter and enjoys home based hobbies much more - he is very outgoing and is a member of loads of activity groups, does outdoors stuff, is very loud and messy. They work together wonderfully and complement each other very well asa couple, but living under the same room was a great strain for both of them. No real 'fault' of either of them tbh, it just didn't work.
They didn't want to split up, they adore each other, they wanted things to go back to 'how it was before' so that's what they did. This was ten years ago and they are still deeply in love, spend a lot of time together, live two streets away from each other, and are very happy. They don't have any children together though, which I think would affect things much more.
So it can work, but I think it depends strongly on the type of people you are, and the reasons why you wish to do this. It wouldn't work for a lot of people after a while, as they would feel like nothing was 'moving forward' maybe? Or that something must be missing from the relationship if they can't live with each other...