Been poking around and have seen nothing like this so far.
When initially purchasing a dual talent spec, the contents of your current action bars are copied into those of the new spec. After that, they are independent.
Has anyone seen an addon that would copy the actions from one spec into the other, essentially duplicating the "initial purchase" copy? Note that I'm not talking about button layout and visibility; talking about the contents of an action button, stored server-side. The addon would have to run through the 120-ish action buttons, remembering their contents, and then after swapping to the other spec, use... what... PickupAction and PlaceAction? to reconstruct the contents of the action bars?