Can you elaborate? Who is filling out the form and how are those two fields determined?

If these are coming from an outside source it would have to be manually entered in, unless this information is stored in a SQL DB or inside of SharePoint. If that's the case you can use the User Profile query (for SharePoint) or the Execute SQL for SQL. 

Our employees are filling out the form.  They need to pick a customer (as a reference) to include on the form.  Right now they just manually type in the Customer Name and the ID number - which adds time because they have to go look it up somewhere else, copy it down, come back to the form and then type it in.  Especially time consuming on mobile app.  

Hoping something might be in the works to enable that kind of lookup from the form itself.

I dont think UserProfile Query or Execute SQL are available in Nintex Forms for O365.

In classic mode we were able to do some hackery with javascript to query locally cached CSV, but there is no javascript on the new Responsive forms. 

Ok I understand what you mean now. My only suggestion would be to store all the customer data in a SharePoint list on the same site and use a list lookup. You can use a cascading list lookup so the ID matches the Customer name. User selects Customer name from a drop down, then the ID would populate automatically. Cascading drop downs and Nintex Forms for SharePoint  Here is an old but good guide. Should still apply to O365 and Responsive Forms. 

My issue in that case becomes quantity, I think Nintex (and SharePoint) lookups have an upper limit -- i want to say 2000, and we are looking at almost 5000+ potential customers to choose from.

If that's the case you'll have to add some additional fields to allow for filters. Can the list be shortened by other parameters? State/City/Type etc? Then you can have a 3-level cascade. Select the sate, then the next dropdown only shows customers from that state.