Quellensteuer is tax deducted at source. It is only designed for one source of income. When you have two incomes you will have to do your taxes for both at the end of the business year.
Normally your gemeinde will just change your standing and may give you a c permit.
Best to go talk to them and also to a book keeper.
I have a salary which has all the normal deductions and tax, and an property rental income. This is only taxed, no social security, etc.
D