In recent years, there has been a trend toward 50-50 custody. Sometimes it is in the best interest of the children, and sometimes it is not.
The key to co-parenting is being flexible and accommodating. So while some families opt for this kind of arrangement, for many it is not the right fit — either because it is difficult for the parents to navigate it, or their schedules preclude it (i.e., one parent travels often and unexpectedly for work).
Generally speaking, a couple can determine a custody arrangement that works for them. If they are unable, the court will determine the status of custody and the schedule.