Without being able to see your return it is impossible to say why this would happen.
What you can do is look at your 1040 with both kids on the return. Write down all the numbers on page 2 or print your return out. Then delete the second kid and then look at the numbers again comparing them to when he is on the return. This will tell you what is changing.
If you need further assistance, please let us know what numbers changed when you add your second kid.
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