My brother in law has apparently been sort of doing a hack with iOS contacts for years with his business. He tutors high school kids, so he would create one contact called, "John and Billy Smith", and then save both the father and son's iPhone numbers in that one contact. When he wanted to message the son alone, he would start a message thread from the phone number, and the similarly with the father, and then a thread with both of the numbers if need be. It seems now that he updated to iOS 13 it treats all messages to the contact as one thread, regardless of the number that you wish to choose.
I explained that while this hack was easier for him for the last bunch of years, it's not how contacts was meant to be used, so he may need to go in and separate all of the contacts that he did this way.
Is there a way to message a contact on one number and then message the same contact on a different number, but keep the threads separate?