There are a lot of resources about how to manage jealousy or other stuff related to your partner having another love. I think the position of, say, a leg in a V (meaning you have one partner who has another partner) is considered to be more difficult to adjust to in our monogamous culture. Which may well be true, on average. But I do think there's quite important adjusting needed of the hinge, i.e. person with two (or more) partners. I can think of a few things that seem to feature in people's experiences.
Taking care of own needs.
There are probably other things that I didn't think of. Please, feel free to add as you think of any.
And please share your experiences about what you have found important/influential.