How do I stop my iPhone from merging text stings from a single contact with two separate iPhones and Apple accounts?

First of all, my iPhone is current with updates (17.5.1).

For married couples, I create a single contact record for them on my iPhone (e.g. John & Jane Doe), and included both of their iPhone numbers. When I add their numbers, I choose "mobile" for one spouse, and "car" for the other. When I text the contact (John & Jane Doe), I choose the dropdown menu and select either "mobile" or "car" depending on which spouse I'm texting. It works great, until it doesn't. Once I choose to text the other spouse by selecting their respective phone ("mobile" or "car"), or if the other spouse texts me, all the texts get merged, and it's difficult to figure out who's receiving or sending texts from/to me.

Most recently, I was texting on a string to one of the spouses, and made sure I chose that spouse's phone from the dropdown menu. But, after a few texts, the spouse I intended to text let me know that his wife was receiving my texts and forwarding them to him. And he notified me on that same steam. So, basically, the iPhone can't distinguish between the two numbers, and merges all texts going to either of them because they're part of the same contact.

Maybe an easier example would be if someone has a single contact with two iPhone numbers (one work, and one personal), and doesn't want texts to those iPhones getting merged because they don't want their personal texts showing up on their work phone, and vice versa, how do they accomplish that. Surely, this isn't the first time someone has brought this to Apple's attention.

In short, I want a way to stop my iPhone from merging texts from different numbers. If I am texting one number for a contact, and then text another number for the same contact, I want two separate strings.

Worth noting, a LONG time ago my texts didn't get merged. This happened at some point following an update, but I couldn't tell you when that was, or what update number it was.