I'm not sure if I understand what you are asking from just a title. But if one person had two partners, you can move a child from one partner to the other by editing who the other parent is. You do that by:
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clicking on the child in question
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clicking on Change or Delete on the left
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Clicking Change parents
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And setting the wrong parent to the correct parent
It does not sound like you need to worry about the secondary parents on that same screeen unless the child was later adopted by the other parent.
