In Agents of SHIELD, Coulson informs us that Simmons has two PhDs in fields he can’t even pronounce. In another example, Tess from Eureka has three PhDs (why would anyone do that to themselves?) I’m sure there are more.
For some reason, there’s this ideas that the more PhDs you have, the smarter you must be.
You don’t need multiple PhDs unless you’re doing a radical career change. It’s like getting a bachelors degree, then going back to school for another four years to get a second one.
You don’t hit a ceiling when you get a PhD. PhD students are not at the top of the hierarchy. When you get your degree, you’re ready to move on to the next step in your natural career progression, which is to become a postdoc.
If you chose to go back and do another PhD, that’s a career change. For one, you would never get a PhD in, for instance, molecular biology then go back and do another one in chemistry. You would do a PhD in either molecular biology or chemistry, and incorporate both interests into your project. It doesn’t matter what the name of your specific PhD is–it matters what you’ve been working on, while you attained the PhD.
Furthermore, if you don’t become a postdoc in, say, chemistry, and instead go work on an astronomy degree, you’re leaving chemistry for astronomy.
Now, if you wanted to study the chemistry of meteorites, or something, you might get a degree in one or the other and find a lab interested in astrochemistry. Or, you could find a lab that is interested in pursuing your project, but has no previous experience with astrochemistry, and set up a collaboration between labs where the two labs can each fill gaps in each others’ knowledge.
The point is, a PhD program is there to teach you how to do research. Once you learn how to do that, you don’t need to go back and be a graduate student again (and if you do, you probably shouldn’t have graduated in the first place).
Multiple PhDs is for drastic career changes, when you can’t get the job you want with what you’ve studied. Otherwise, you get a job, and maybe even make strides towards having your own lab one day.