Co-op + platformer is an ancient combination. The earliest one I remember playing was the first Toe Jam and Earl for the Sega Genesis. Since then it has been done and done again. As such it can be hard to make good on any new entry in the genre. But Knack II seems to do a decent job.
My partner and I played as Red Knack and Blue Knack. The level we played was mostly platforming so it provided a real good feeling for how the exploration mechanic works. Just as in the previous game you can swap Knack from large to small and you can double jump. But that it's not the best feature. The best feature for co-op platforming is the teleport to my friend button. If you cannot make a jump or are falling from a ledge you can quickly tap R2 and be teleported to your friend. I know in past platformers if one player got stuck on a spot it could be a real drag while waiting for them to catch up, or having the fight of "jump now, no double jump, no not that way, dammit just give me the controller". No one really wants that.
Combat was also pretty fun. It was mostly punching, but you can use your cohort to initiate some fun stuff. The two we combos we could do was machine gun and an AOE. The machine gun involved standing behind the other Knack and punching, a lot. That essentially sent parts of him flying forward. The other one was jumping over your friend and doing a belly flop. This sent his parts flying in all directions. Both of these were fun as your associate would get his parts back when the attack was done, and who didn't want to attack a friend and get positive benefits?
Overall the demo was really fun and brought back my child good of a simpler time. I am really looking forward to Knack II on September 5th.