This is easy and yet complex. I was under the impression that one playing with multiple sports is one nomination. A group of distinct separated playing fields can be individual nominations.
In theory. Niantic Criteria indicate that a multisport field be nominated as a multisport field - listing the sports.
This is a common problem from the UK to Australia. And each nomination waits for the sport season or when council changes goal posts. So one field ( the same field) has nominations for cricket , football , rugby field and if Australian - AFL/NRL as well etc etc. And people walk to other side of the pitch and nominate that “goal post”.
I quote from Niantic and see if that helps 
Sports fields and athletic fields encourage exercise and socialization and are often good places to explore. It is important to choose a pin placement that respects and does not interfere with the sporting activity. See the image below for guidance. For sports fields with multiple points of play, there should be one nomination to encapsulate the entire field.
When I review. I err on the side of caution and very much look for PIN placement and see for other “related” nominations to see if the opposite goal post is nominated etc.
Good luck and have fun out there