How do you introduce your partners to each other?
Slowly, in low-stakes contexts, with no pressure for them to like each other. Start with a brief casual meeting; let any closeness develop on its own. Don't force kitchen-table dynamics that the people in question don't want.
Introducing partners to each other is a particular sub-skill of polyamorous practice. The fantasy version is a warm dinner where everyone bonds; the practical version is more like introducing colleagues from different parts of your life and seeing what happens.
Standard sequencing. Tell each partner about the other before any meeting — give them a sense of who the other is, what the relationship is like, anything that might come up. Plan a first meeting that is low-stakes and time-limited: a coffee, a brief drink, attendance at the same event with no expectation of sustained one-on-one time. Don't load it with significance.
What to watch for. Each partner has their own pace for these meetings. One may be eager; the other reluctant. The eager one's pace should not set the reluctant one's. The pace of integration is the pace of whoever is most cautious. Forcing a faster pace usually creates friction that wasn't otherwise present.
What works. Letting any friendship develop on its own. Not relaying messages between them after the introduction; let them have a direct relationship if they want one and a no-relationship if they don't. Not making it your project to get them to like each other. Accepting that many metamours end up at cordial-but-distant — and that's fine.
What does not work. Big-event introductions where both partners are seeing each other for the first time in front of an audience. Pressuring either of them to be closer than they want to be. Treating the introduction as a test the meeting either passed or failed.