Hi,
As unmarried/ non-civil partners, you may decide the percentage split between yourselves.
In terms of a bank account, this does not need to be considered and is determined by the party who claims both income and expenses of the split.
Although there is no issue with deciding to start with a previous year, we would expect the income to correspond to reality as per the following guidance, and the declarations should reflect this:
TSEM9160 - Ownership and income tax: legal background: ownership - income follows property
Thank you.