The short answer to this is that you cannot simply copy the silkscreen from one side to the other. The longer answer is that you cannot simply copy the silkscreen from one side to the other because most of the silkscreen is directly associated with the Footprints so you cannot copy, paste or delete one without the other. And because the silkscreen is associated with the footprints, you cannot mirror any of it, you can only rotate it. You cannot simply copy it and the place it into the bottom layer because all the footprints will appear mirrored because they have been flipped over onto their backs to fit on the underside of the board.

if it is not possible now, so be it. I can do it manually. I think silk on both sides will bring value for debug, assembly and more. I also think that it is doable, because the silk layer doesn't carry any electrical properties, so it should be doable to automatically copy the silk layer, mirror it, and mirror back the characters. Still someone has to do it.