There are two ways to split a field. The first is based on splitting hectares in the area box.
When adding a new crop, click in the Area box and input the required amount of space for the first crop:
Then press Save.
When adding the second crop to the same field the remaining area will auto populate:
The second available process is based on inputting the crop boundary, giving you a visual guide on how the field is set out on the map screen.
When adding your new crop, select Crop Boundary:
Select your boundary by clicking/dragging the points onto the map and, when finished, click OK in the top right:
The used hectares will populate in the area box of the add crop screen, you can then click Save:
Repeat this process to add a second crop, they will then be shown as separate on the map screen.
Assuming the crops are different they will show as two different colours when viewed on the crops map screen: