I want to filter collection for same attribute having two either values.

example:

->addFieldToFilter('customer_group_id',Mage::getSingleton('customer/session')->getCustomer()->getGroupId())   


->addFieldToFilter('sales_org',Mage::getSingleton('customer/session')->getCustomer()->getSalesOrgId())
              

->addFieldToFilter('sales_office',Mage::getSingleton('customer/session')->getCustomer()->getSalesOfficeId())

My question is. If cutomer_group_id == 12 then it should have sales_org and sales_office value. else if customer_group_id == 13 then it should be null in sales_org and sales_office.

How can i achieve this filter.

Thanks in advance.