If you have a supplier where you need to split the charges on their documents across more than one nominal ledger code, you can add more than one nominal ledger code to their supplier rules and set a percentage on each code. See examples below. 

The total goods value will then be split across the nominal ledger codes you selected at the percentage you chose to split them. 

Please note, the percentage split will automatically be 50%/50%. If you want to edit this to be anything other than 50%/50% you will need to be sure the percentage adds up to 100%.