In our defence, combined dynamics are not intended for representing dynamics that change on different passes through the music. They are instead intended for rapid or instantaneous changes of dynamic on the attack of a note, such as fp or pf. In our survey of the literature we didn’t come across these kinds of rapid changes of dynamic from e.g. f to mf or p to mp. So the data structure was designed in such a way that one side of the dynamic would be forte and the other piano. It’s not some great conspiracy to deliberately limit or hobble the program, and I believe the design we came up with is appropriate for the kind of dynamic for which it is intended to be used, though I daresay somebody can provide me with one or more counter-examples from the literature that show an accent that goes f-mf or mp-p or something if they look hard enough.
There is, however, a separate need for dynamics that differ on different passes through the music, and that’s something that Dorico doesn’t yet properly support. Using a combined dynamic for this is really a workaround until such time as we have a proper feature for it.