This just in from The Dead Weather mailing list:

Due to overwhelming demand, a second show has been added at Terminal 5 in New York City on July 17th. A limited number of [Music Today] presale tickets will be available tomorrow, May 27th starting at 10 am EST HERE. Tickets go on sale to the public on May 29th at 12 pm EST.

There is also an Amex presale that starts tomorrow (May 27) at 10 AM.