I uploaded a combo MOBI/KF8 ebook quite a while ago.
I don't really have much of a vested interest in how it ultimately gets handled, but there's something to be said for the fact that uploading combo files means nothing has to change from the current forum setup: no new subforum... nothing. The MOBIs in the MOBI forum would just slowly start gaining KF8 functionality. Those who know and care can always do whatever they want to the file anyway... and those who don't care won't have to mess around with inevitably downloading the wrong format and getting all cranky about it. Why reinvent the wheel for stand-alone KF8 ebooks?
Also, I believe if a post has two attachments (one for mobi and one for azw3), it could possibly affect calibre's Get Books search feature.
However it turns out... I believe one ebook attachment per post should be part of the procedure.