This is based on WA law and, again, IANAL:
Support is based upon the combined income of the parents of the child and the number of dependent children each parent has. Generally speaking, the courts do not take bread from the mouth of one kid to feed another. What the court should do, if asked, is modify the ordered support amount based upon Jim having two dependent children–after kid 2 is born. Until kid 2 is born, there is only one dependent child to consider. But the court can only do this if asked, so Jim (either on his own or via his attorney) will have to go to court and request a modification. There are things that can be used to deviate from the standard support amounts that are set as well–travel costs, insurance costs, voluntary un/under-employment of one parent, etc.
Setting child support can be very difficult, even when the parents get along and are working together to come up with something equitable. There are added challenges when the parents don’t agree–I hear about them every day.