What to expect when pursuing a second doctorate in an unrelated field
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!