I discovered that the hard way.
Eventually I opened 1040-X in the Forms view and hand entered numbers from my original filing, the first amended filing, and the 1040 being computed for the second amended filing. You need numbers from all three to get it right. Specifically tax owed (or refunds) for all three scenarios in order to get the final bill/refund right.
The advantage of this approach is that this 1040-X is in the Turbotax world, so you can re-file the package electronically. Using the Federal PDF would mean going to USPS.
