Not only is this a fantastic co-op mod even in this early state, you can actually punch each other and do special moves on each other, which is far more than I expected to be possible for a co-op mod. I give it a "Wah" out of 10, the highest praise I can think of.
Viewing post in Wario World Multiplayer (Alpha) comments
Adding onto this after further messing-around with a buddy of mine. This is probably known by now, but just in case it isn't, there's a couple of bugs we found:
First off, the trapdoor just before the boss door in Greenhorn Forest crashes the game when trying to load the jewel challenge area, every time. There's probably other ones too, but my friend and I jumped to just fighting the main world bosses after that happened.
Second, Player 2 cannot interact with breakable scenery or anything that requires doing a special move on an enemy to use, like the spin wheel to open the first gate in Greenhorn Ruins.
Lastly, the Black Jewel fight completely breaks if Player 2 hits the trapped sprites, since they attack P2 instead of the Jewel and then never respawn no matter what you do, forcing you to back out to the main menu
I hope the feedback helps, since this is a fantastic bit of wizardry you've managed, and I'm looking forward to seeing it develop further.