I added one small W2 and the tax amount changed by $400, what is incorrect?

It depends.

Did you have any withholdings on the W-2 for federal and state taxes?

If not, that could be a part of the issue as your income may not have been taxed. 

Are you filling out an original return or amending a return you filed?

That's common. It just looks that way because you put them in as separate W2s and saw the tax due change in between them. If it all was on one W2 you would get the same answer. And each job was withholding like it was your only job.

Because you only get one standard deduction no matter how many W2s you put in. Turbo Tax starts out by giving you the Standard Deduction. You entered more income when you entered the second W2 but you didn't enter more deductions. And each job only withheld taxes like it was your only job for the year. You might want to adjust your withholding. Also as you add more income you might not be getting as many credits as before like the EIC credit. And it might have been giving you the EIC credit or some other credit until you went over.

See this FAQ
https://ttlc.intuit.com/community/refunds/help/why-did-my-refund-drop-when-i-entered-a-second-w-2/00/26275