What to expect when pursuing a second doctorate in an unrelated field Long story short: I got a PhD a few years ago (let's say, in numerical methods for nonlinear PDEs).  I've since gotten experience with a couple post-docs and a variety of collaborators, but I'm not really satisfied with what I'm doing.  I feel out of place.

Earlier in life, I made a tough decision between science and humanities-- my other passion has always been, let's say, east Asian languages.  If my bachelors program had had better language courses or options for "hybrid" fields like computational linguistics, I probably would have gone that route, instead of fully focusing on math.

I've sought out collaborations that might help me bridge the gap between math and languages; I've also looked at postdocs to help make the transition.  I've reached out to leaders at research groups who do stuff I'm really interested in.  The common denominator is: I'd need a PhD in some kind of linguistics to research seriously what I want. So...

What key things should I take into consideration when pursuing a doctorate, given that I have a doctorate in an unrelated field?

Please consider the question within the following context: I'm not asking whether having two PhDs is good or bad (I frankly don't care), how to needlessly pad my name with titles, or how to get two PhDs simultaneously. (That is all to say: this post doesn't answer my question.)  I'm asking about what to expect when changing careers, specifically, if the source career is in one academic field and the target career is in another, unrelated field.  Assume I've 100% decided to leave my current field and pursue this second doctorate, for the purpose of changing careers. Assume also that I already have substantial content knowledge in the target field, as I have a bachelors degree in it already, and it's been an avid hobby for most of my life.

Some sub-questions that don't need to be specifically answered, but I think help narrow the question: in what way could my current PhD be a hindrance in getting into a new program?  Will my supervisors and colleagues see my past PhD as an asset, or as weird-looking mole they will try to politely ignore?  What are other questions should I ask myself to prepare for such a non-traditional career path?

Note: I've edited this question multiple times in a vain attempt to get the duplicate label removed. If you agree, please vote.
In my experience, nonstandard paths like what you propose are initially viewed with surprise, and with varying degrees of suspicion. That includes admission committees, potential supervisors, fellow students, etc. However, that comes from uncertainty and unfamiliarity and can be shaped by the narrative you supply, and how you follow through.

Basically, people will probably have 3 archetypes in mind when you first contact them. The most positive one is that you are some sort of polymath genius. The most negative one is that you are a serial fail-to-deploy-in-life perpetual student, a model everyone in academia has encountered. And the 3rd is that you want to do research in some unexpected interesting intersection of your old and new field (which you've said is not the case). You therefore need to replace that narrative with a truer, credible, if possibly slightly airbrushed one, something like: you've always been interested in both X and Y, initially pursued interesting topics in X leading to a Ph.D. and start of a promising career, but are increasingly passionate about Y and want to refocus on that. I would be open that you don't expect to directly harness your expertise in X, but that you do expect that your background will help you be a very effective student in Y.

And then you need to back that up with your actions! Your Ph.D. is a credential indicating you have demonstrated the ability to advance the state of the art of knowledge in some field, and that you have learned sufficient academic "tradecraft" to be an independent researcher, at least from a competence point of view. (Footnote: The at-best only partial independence of a post-doc, and indeed the whole post-doc position, is more a reflection of funding realities than of a junior academic's presumed research abilities). So if someone with a Ph.D. and post-docs would approach me, I would expect them to be able to motor through the usual initial stages of a Ph.D. much more quickly than a "new" graduate student. I would expect that they will have done a fair amount of foundational reading in the field already -- that's something that hardly needs an advisor, though an advisor will be helpful in helping identify important strands that may be nonobvious from the outside. And I would expect them to arrive with some well-developed thoughts on potential research topic, though of course we might well adjust it together.

If you were to come with this type of pre-preparation, I'd consider you a lower-risk admit -- as a committee member or prospective advisor -- than someone whose basic skilset is unproven. Conversely, if you were to arrive, with the background you describe, with the typical keen-but-blank-slate mindset a fresh-out-of-undergrad student usually brings, then I would be more worried you belong to the serial-failure-to-deploy archetype instead.

Second, I would suggest you be very open about discussing your financial expectations. Do you need/expect funding (and is it usual in your target field)? Or can you fund yourself by continuing to work in your current field part-time? Or do you have enough saved up from a well-paid science/tech job (and academics in the humanities/social sciences tend to assume all science/tech jobs are well paid....) that you will be self-supporting? All of those are acceptable, just avoid unpleasant surprises from failing to bring up the topic.

Good luck!