This article will give you step by step instructions on how to configure weekly + monthly payroll frequency for your clients.
Adding the lookup
Go to Settings > Lookups > Payroll frequency > Add item > Type in Weekly + Monthly Payroll in the empty field > Click save
Editing the Payroll details form
Settings > Forms > Payroll details > Date of next payroll > Change the field name from "date of next payroll" to "date of next payroll (non-weekly)"
Click on add filter > Scroll down to payroll details > Payroll frequency > Click OK > Tick as follows: weekly + monthly, monthly, quarterly, fortnightly, four-weekly, six-monthly, annual (but NOT weekly)
Click on Add new field at the bottom of the page and add the field name "Date of next payroll (weekly)", make sure to select the "date" field
Click on Add filter > Payroll details > Payroll frequency and select: weekly and weekly + monthly