Hi,
Both incomes will be treated separately with regards to Income Tax and have 2 seperate tax codes allocated. The personal allowances for the current tax year are £12570, if you earn above this with your employer any secondary income will be taxed at 20% or 40% depending on your total income for the year.
You can below a link to the Income Tax rates to determine your highest rate of income tax.
Income Tax rates and Personal Allowances
Sometimes a tax code at your main source can be reduced for various different reasons, to determine the reason for this, you would need to contact our Income Tax team to review your records.
Income Tax: general enquiries
Thank you.