When they build lines they must have at least 48hrs off in base before a change in direction (Trans-pacific to SA or Trans-Atlantic). I would have to do some more digging to see if there is a similar restriction in the construction of trips. In my experience (JFK ER) this does not happen. Trips that have multipule crossings are almost always to the same forign destinaion (JFK-AMS-CVG-AMS-JFK for example).