As a med student, I'd have to agree that this is very odd. When you touch someone's prostate, it feels very soft, as if you were touching a sponge through a sheet.
In other words, I guess you would not be able to feel a prostate when penetrating with a dick unless your cock was very sensitive on the "body" (as opposed to the "head"), which people tell me is not the case.
If you did had this oversensitive dick, you really be able to feel the prostate in both the vagina as in the rectum, because the wall that separates them is very thin, based in the regular female anatomy. If anybody want precision, take a look in this anatomically correct image from one of my books:
https://www.netterimages.com/images/vpv/000/000/007/7237-0550x0475.jpg
Since we can only speculate how a futanari-herm anatomy would be, I'd say that volume saving would be a priority on a creature with so many organs in such a small space (either that or futas would have to lose some of her bones/muscles/blood vessels in the hip region to accomodate all), so I'd say that it would be even more easy to feel a futa prostate through her vagina and her rectum.
Yes, anal lovers, you read right: a futa would probably have more accesible (and, why not, more sensitive) prostate than a man would. More anal pleasure? Maybe (Probably?)