why not that's a big pot, i say just get a big old piece of plastic, divide the pot in two then grow! just use string to pull the plants into a V as they grow so there's lots of air and light inbetween the two
cardboard is gonna saok away into crap in no time after a few waterings. ideally it should be something waterproof as well, as chances are the two plants will drink different quanities.
even if clones, with a divider, you're just getting rid of a slight posibility of them getting all root tangled and such, and say one turns up to be male, hermie, a bad plant etc, and you've got to pull it, you're not gonna have a fun time doing that if you've got the other ones roots interlocked with that one