So there are already some good questions on the topic of double doctorates:
I'm looking for some advice specific to my situation. I'm currently enrolled in a Ph.D. program in a mathematical field (computer science) but most of my research is geared towards applying these methods in a scientific field (stellar astrophysics). Because of this, my advisor is encouraging me to take up a second Ph.D. in astro. He says I can write one dissertation to get both degrees, and he doesn't think that it will delay my time to graduation too significantly.
On the one hand, it is attractive to me, because it might help qualify me more towards future positions in astrophysics. On the other hand, I don't necessarily see too much additional value in getting a second one; to me a Ph.D. is a license to do research, and one doesn't need two licenses. Further, I'm sure there is a real risk of it delaying my time to graduation significantly despite what I might be told. I'm also not completely convinced that it would equip me with skills that I wouldn't already be getting, although it might demonstrate to others that I have those skills. But then again, can't they just look at my publication record? Finally, while I absolutely love astrophysics, I am also attracted to other sciences, and I wouldn't want a doctorate in astro to cause people to think that I can't work in other sciences also.
Are all these concerns legitimate? Have I laid out the pros and cons appropriately? Are there other considerations I should contemplate? Are there some drawbacks or advantages that I haven't listed?