Hi,
You can't unfortunately file a Form 12 online, has to be paper filed and sent to the Collector-General.
If the property is in joint names you should split the rent and expenses evenly between them. This is especially important if they made a profit and where their other income does not allow them to claim the full 25k standard rate band increase.
Depending on the source of their other income it may also impact of their PRSI/Levy liabilities and be better to spilt the income.
Bailey