Hahaha seems like you have a lot of bad ideas
On a serious note though, I think it's kind of an over-the-top idea... You could put two 6-string fretboards together and make a 12-string one, but two actual necks put next to each other would be just... Well... Why? Just buy a chunk of wood and shape it to the size and shape of a 12-string neck
If you do decide to go ahead with that project though, please let me know
Same goes for the 9-string idea... Which is an idea that, as outrageous as I consider it to be, I do support it.