Metamours, your partner's partners, are one of the more consistently underwritten relationships in CNM. The attention in polyamory communities goes to partner relationships and to jealousy management. The metamour relationship is often treated as a consequence of non-monogamy rather than a relationship in its own right that benefits from intentional approach.
In practice, your relationship with your metamours has significant effects on your experience of CNM. It's worth thinking about deliberately.
You didn't choose them
The first complication: unlike your partners, you didn't choose your metamours. They're in your life because someone you love fell for them. You might find them genuinely appealing; you might find them difficult; you might feel neutral. Whatever you feel, they're part of your relationship landscape whether you sought them out or not.
This is worth acknowledging because it sets realistic expectations. The pressure to like or be close to your metamours, which exists in some CNM communities, particularly in kitchen table polyamory contexts, isn't always realistic. Genuine warmth is great when it exists. Polite respect is the functional minimum. Forced closeness with someone you genuinely don't like tends to produce a different problem than the one it was meant to solve.
The kitchen table vs parallel choice
Your CNM style shapes what you're actually trying to do with your metamours. In kitchen table polyamory, the goal is genuine connection, knowing each other, spending time together, being part of the same social world. In parallel polyamory, the goal is functional coexistence without significant overlap.
Most people's practice lands somewhere between these poles, and it can vary by metamour. You might be genuinely close with one partner's other partner and maintain comfortable distance from another. This is normal. What creates friction is when your preferred approach and your metamour's (or your partner's) preferred approach are significantly mismatched, particularly when one person wants more closeness than the other is comfortable with.
What actually helps the relationship work
Basic mutual good faith. You don't have to be friends. You do need to start from the assumption that this person is not a threat and is not your enemy. They're someone who loves your partner. You love your partner. That's actually a point of common ground, even if it's also a source of complexity.
Not treating them as competition. The instinct to compare yourself to a metamour, to measure whether you have more of your partner's time, whether they're more attractive, whether the connection with them is more exciting, is understandable and almost always counterproductive. You're not competing for a fixed resource. Your partner's care for someone else doesn't reduce what's available for you, and treating it like it does produces the kind of anxiety and behaviour that damages relationships.
Direct communication when there's a problem. If you have a specific issue with a metamour, they've done something that directly affects you, there's a pattern that's creating friction, the most useful approach is often to address it directly with them, or to ask your shared partner to facilitate a conversation, rather than processing exclusively through your partner. Processing through your partner consistently puts them in a triangle they didn't ask for and rarely resolves the underlying thing.
Not asking your partner to manage the relationship for you. Your relationship with your metamour is, to some degree, yours to build or not. Expecting your partner to maintain your metamour relationship on your behalf, to reassure you about them, to mediate conflicts, to translate their behaviour for you, puts a burden on the shared partner that isn't theirs to carry.
When a metamour does something that affects you
Sometimes metamours do things that have direct effects on your relationship with your shared partner, they ask for more time, propose changes to existing agreements, or have needs that create constraints. How you respond to this matters.
The version that tends to go well: treating their needs as legitimate even when they create inconvenience for you, expressing your own needs clearly to your partner without framing the metamour as the problem, and trusting your partner to navigate the competing needs of their relationships with some skill.
The version that tends to go badly: treating every metamour need that affects you as an imposition or an act of aggression, expecting your partner to prioritise your comfort over their other partner's legitimate needs as a standing matter, or making the metamour the explanation for every difficulty in your relationship.
When you genuinely don't like them
Sometimes a metamour is someone you genuinely don't like, not as competition, not as threat, but just as a person whose values or behaviour you find objectionable.
If the dislike is significant enough to affect your wellbeing or the relationship, it's worth naming to your partner as a real thing, "I find them difficult and I want to be honest about that", without demanding they act on it. Your partner's relationships are their own. You're not required to socialise with someone you dislike. You are expected to manage the effect of your dislike on your shared relationship rather than making it your partner's responsibility to fix.
There's a small category of situations where a metamour is behaving in genuinely harmful ways, not just "I don't like them" but specific behaviour that's actually damaging to you or the relationship. This is worth raising explicitly and specifically, in terms of the behaviour rather than the person. "I'm uncomfortable with X thing they do that directly affects me" is a conversation worth having. "I don't like who they are" generally isn't.
The best metamour relationships
The best metamour relationships in CNM tend to share some features: mutual respect for each other's place in their shared partner's life; a genuine wish for each other's wellbeing even without closeness; the ability to communicate directly about things that need communicating rather than routing everything through the shared partner; and the security to not treat each other as competition.
This is achievable more often than the difficulty of the situation might suggest. The fact that you both love the same person, and that person has chosen to be in relationship with both of you, is actually a reasonable basis for goodwill, when it isn't being distorted by insecurity or unmet needs.