Did you check to see if Sonic Generations demo still shows up in your download list? You should still be able to download from there if so. (I assume this isn't a 60 minute trial version that Plus subs are entitled to, because once you download it and use it, that's it.)
You definitely can't transfer the demo through the PS Plus cloud storage regardless.
The only other thing is to do a full back up of the PS3 that has the demo, and then restore it to the second PS3, essentially making it a clone of the first and wiping out everything else on it.
Ultimately, it may be easier just to buy the game.