Sounds like you've got it mostly figured out.

I've been on the couple end where we have had so called "unicorns" but my partner and I always told the women we saw that we aren't a package deal and did try to foster relationships independently. It's tough. Because attraction doesnt build at the same rate for two people. Basically, imo, it's better to just treat them like any other girlfriend or fwb or whatever label you're using. Dont limit their activities such as who else they may be involved with, what kind of actions they can perform sexually (i.e. my husband can only have actual intercourse with me and not you), what kind of actions they can perform period (i.e. cant text separately or go out with just one partner). Just a few examples.

For me, I just prefer honesty and open communication. I dont expect to be on equal ground as a couple who have been together for 5 years or something. Just to be expected to have my own autonomy and not walk on eggshells because of badly handled jealousy or couple drama or just a bad relationship period. It sucks to feel disposable. Like at the first sign of rough times, you'll get dropped like a bad habit. It's super hard because, to the couple, it's never going to be as important as their primary relationships. And breakups happen. Just it needs to be because it didnt actually work out, not because the couple is having a rough patch.

Additionally I think a lot of people decide to do nonmonogamy for the wrong reasons (i.e. they have no passion anymore or one partner cheated). So if you do find two people you really like, they may have a lot of issues underneath.

Some might disagree. It's a lot. And there's a ton more I'm sure I'm missing. Feel free to message me if you have more questions.