Man of Two Worlds may be the worst book signed by Frank Herbert that I've ever read. It features that nasty '50s newsroom trope, where single minded egotistical people (sometimes to the point of cruelty) get a pass because they are brilliant and they talk fast (and they own the newspaper). The characters are unsympathetic and rather not interesting, while the ideas in the book are dull and going nowhere. The plot itself is often inconsistent. I do suspect that this is more of a Brian than a Frank book, because none of the themes found in previous Frank Herbert books are found here (if one discounts the awkward depiction of women). Also the tiny Wikipedia page dedicated to the book lists Brian as the main author.
In short, the story is about a human and an alien merging accidentally and having to work together to get things done. Kind of like the authors were trying to do, huh? The setting is classic '50s sci-fi, with people living on Mars and Venus and going from planet to planet using vehicles that function like normal airlines, only in space. Meanwhile, the aliens are trying to destroy Earth because they fear humans - not sure how that would work since they already live on other planets, and the humans have their own familial issues to resolve - including an uncle with god-like powers but no apparent care about the outcome of anything he does.
There are many issues with the book that I am not going to go into. I almost did not finish it. However, someone else might enjoy it, so no need to rant in this review. Suffice to say that the book feels insulting when it is not boring, which is most of the time. I don't recommend it.