Mapping two checkBoxlists in asp.net

I Have 2 CheckBoxlists one is section and other one is division at default page load i will load the section list and when i select the specific sections i need to load the only classes that are mapped with the division

Now i am getting only one sections classes i.e when selecting KG Section the Class list displays the LKG and UKG

I need to diplay classes for multiple Section selections