Hello Mandy,

thanks for reaching out to us.

You can do this with the Special Days option. You can define every 2nd Thursday of each month for Employee 1, and every 4th Thursday for Employee 2.

For example - 

On the booking form only these days are available - 

Just a note - you need to exclude standard Work Hours for both Employees in order to show just Special Days.

If you have any other questions, i would like to help.