My MIL/FIL mostly fit this description - they step up and are there for each other in difficult times (family bereavement etc) but pretty much live separate lives. They won't even sit next to each other on the bus!
It's the 'married for life' culture really isn't it - some people just do not believe in divorce/separation at all and would rather live separate lives within the marriage.
Luckily this doesn't seem to have affected DH too much but we have our own issues which we are waiting counselling for.
LeQueen similar situation with inlaws. fil had another woman for years - if there mil cooks meal - he eats it - makes stupid comments and then takes out the OW. We took mil out for meal a few years ago - the first time she'd ever been out for meal. Last 8 years fil lived with OW in the family hoouse - mil just bowed out and stayed elsewhere.
From the outside looks like normal family but really not - mil was very active in this lie - and is a real martyr and controls her adult kids to within an inch of their life - the kids relationship with their dad was very muched based on his treatment of mil - not great - has affected my marriage as I cannot be a martyr mum.
Fil passed away earlier this year - mil and family inherited everything and they feel everything is now in order.
Each family has their own dynamic - I found them very tough - they found it normal and didn't know any different and would never have contemplated divorce.