Bajablo on 04 January 2013
The wiiU and the 3ds can speak to eachother via IP/ethernet already, so for cross-play you could just use that. The latency shouldn't be an issue if you have a 802.11n wireless network (depending if you have alot of wireless devices).
It is only if you want to display what the wiiU's rendering that you'd have to use it's wireless GPU output and have the appropriate interface on the 3ds (expansion card).
