So I am an undergraduate in university right now studying applied physics with applications in aeronautical/aerospace engineering. I have a 5th year M.S option to get my masters in physics with applications in a certain field. After that 5th year M.S I want to go and get my masters in aeronautical/aerospace engineering but always wanted to do research at the PhD level, so is it possible for me to get a PhD in physics or aerospace/aeronautical engineering after two MS's in physics and aeronautical engineering respectively?
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.
I did exactly this, just with some differences in the subjects. I did my Masters degrees in Education and Physics and had no problem getting into the PhD in Physics. I am fortunate, as where I am, students are not required to pay for research degrees (Australia).
One major advantage I found was that the increased research skills benefited me in the PhD, this is what helped me when I was putting together my research proposal which my University's selection panel focused on.
If you can, go for it! As always though, speak with your advisor - it may be unnecessary.