Conditional Drop down from two possible related tables?

Alright, so I am needing to create a conditional drop down selection of contacts based on what parent company is selected. The catch is, the two company types are related to the contacts table separately as parent tables. 

For example:

Say I have a complaint. If the complaint is of type "Vendor Complaint" and a Vendor company is selected, then I need to see a drop down selection of contacts with parent Vendor matching selected. But if type is "Customer Complaint" and a customer is selected, I need to see a conditional selection of contacts with parent customer matching selection. 


I cant figure out a way to dynamically change what the conditional drop drop is narrowed by. 

Any help is appreciated. Thanks!