At present, Invention is still a recent game update, so there's some overlap between what fits on the two forums. It's not a bad thing to encourage the enthusiasm and interest by leaving those threads where they are, since they are for the most part feedback.
In due course, I would expect that active Invention threads in RGU will be moved over, but I don't think there's any need to do that just yet.
~~~~ Just another victim of the ambient morality ~~~~