Multiple Enterprise Developer License account for a single DUNS number
I haven’t seen this ever. Most companies end up with too many DUNS numbers and not too many accounts on one. 

I would recommend you guide your clients to make their own developer account - one for each business in their name, with their DUNS and their ultimate control. Have them add your single developer AppleID be added to their accounts. That scales well, protects you and your clients and let’s your team work on different accounts. Then you are an authorized agent on all of the clients that subcontract you to developer and provide them with services.

I do think you’re trying to do the right thing separating the accounts, just that each company should have its proper address entered correctly. 

Your contract lawyer would be the best person to advise you since I could see you jeopardizing all the accounts if Apple determined you impersonated other entities or didn’t respond legally correct information on the contract you are signing as a developer. 



Specifically, and not even considering getting the accounts approved in the first place, you would likely run afoul of 4.2.6 app review guidelines unless all your clients had the same model and you just submitted one app that aggregates in the “picker” model.


  4.2.6 Apps created from a commercialized template or app generation service will be rejected unless they are submitted directly by the provider of the app’s content. These services should not submit apps on behalf of their clients and should offer tools that let their clients create customized, innovative apps that provide unique customer experiences. Another acceptable option for template providers is to create a single binary to host all client content in an aggregated or “picker” model, for example as a restaurant finder app with separate customized entries or pages for each client restaurant, or as an event app with separate entries for each client event.



https://developer.apple.com/app-store/review/guidelines/


There it explicitly warns that the provider of the content should directly submit their apps and that your role is to provide them the tools to submit their own app. If they subcontract that to you, a good contract lawyer can keep you set both as a company, for your clients and to satisfy Apple if asked about the relationship.