Hi,
As you both own the property you both need to declare the rental income received unless you have supplied an alternative deed of trust as to who is the owner of the beneficial income.
You are correct in that your personal allowances will be used against your self employment first.
As the mortgage is now given as a relief, this is restricted to 20% on the profit made and if it covers the actual tax due on the profit then nothing else would be due on the property income. You can see guidance here:
https://www.gov.uk/guidance/income-tax-when-you-rent-out-a-property-case-studies
Thank you.