Only way I know of is....

Delete the data you have for it like planting, harvest, etc maps and the large single field. Assuming you have that data yet on a stick or in the files page if you look back far enough. 

Make your two boundaries as they should look in their separate new fields. Make sure you have the boundary sort option turned on in your OC settings. 

Re-import or reprocess on the Files page the data for them and it should sort/split them when it's bringing that stuff back in