Getting a PhD after two masters?
If you can afford to do all that graduate study without a TA-ship or RA-ship, more power to you!  Perhaps you could get the two masters concurrently... or at least get some of the coursework for the aeronautical/aerospace engineering masters out of the way.  Maybe there will be some nice overlap that will speed things up a bit for you.

You could also take those a/a courses in the early stages of a PhD program in either field.