If both transaction hits same transaction key, then you can't substitution the amount into two different G/L account. If you change the G/L account to 520430, then all gain and loss amounts will goes to this new G/L account.
I hope you need a badi or user exit to achieve this. You can hit G/L 520430 for your this type of transaction and rest of all transaction will be hit on G/L 611400.
