How to support complex query with conditional dropdowns

I think that this is easy.
Create a field in Contacts which concatenates the Company and Customer like this

Company-Customer

The when creating an issue have the user select a drop down of which Company and then which Customer and create the same concatenated formula field in the issue and make sure its on the form, even if in a hidden field or section.

Then base the CDD configuration for Related Contact on matching Company-Customer.