Calculating child support payments

This is a difficult question to answer without further information. Where are the children living? If they are both living with the father then the two mothers would each owe child support for their birth child based on income. However it can be more complicated if one of the mothers lived with both children and stood in "the place of a parent" for the other child as well. This question really does require much more information as the issues are and can be complex. I urge you to speak to a family law lawyer to provide further information and give you proper advice based on all of the facts of your case.