One online shop offers 2 wii balance boards as a package, but I found information on wiki that two balance boards cannot be used at the same time. So is there a reason why they offer 2 in a package? Are there any multiplayer games for wii balance board?
There are currently no games supporting multiple Balance boards, and I don't expect there to be any, if only for the price of the devices, and the amount of space needed for such a setup.
All current games supporting multiplayer with a single balanceboard do so in a hot-seat style. (or should I say hot-board)
Technically, for as far as I can see, the Wii itself is perfectly capable of supporting multiple balance boards, but I don't know if this functionality is even availble for developpers
I don't think it is technically possible to have a multiplayer game where both players use their own balance board.
I sadly can't recall where I read this (it's been a while), but I believe that internally a balance board is seen by the Wii as multiple controllers (maybe 2-3 per board). Since the Wii can only handle up to 4 controllers at a time, there aren't enough "slots" to support multiple balance boards.
The reason that Punch-out can't do multiplayer with balance boards is that if the balance board takes up two controller slots, and then the Wiimote/nunchuck takes up another, that's 3 controllers for one player.