How to put multiple entity fields inline as sub fields?
Have you considered Paragraphs? It allows you to 1) create predefined sets of fields and 2) allows you to add fields that allow you to add those predefined fields. You have the option to restrict the paragraph selection as well as the number you can add.

A closely identical module is Field Collection although their project page suggests moving to Paragraphs instead.

It's not the two dropdowns you're looking for, but allows you to select a type and add as many as you want.