No, it's not possible (currently?) to transfer bought games from one WiiU to another.
All bought titles are tight to the WiiU itself, not the user who bought it. When you buy a title, it's accessible by all users registered on that console, making it "for the console" and not owned by the user who bought it.
Allowing transfer of titles from one user on a WiiU to another WiiU would allow sharing titles over multiple WiiU by registering that same user again and again on all existing WiiUs.
Maybe the titles are owned by the user who downloaded the title and these titles can be "moved" (instead of shared) like Wii to WiiU works, removing everything from one user on WiiU1 (over USB for example, making a TransferAccount folder on their own FS with save files and user's data) and putting/adding it back on WiiU2, removing the titles permanently in a way that other users on WiiU1 can't play them anymore. That "could" be done in the future if Nintendo paired bought content to user account instead of console.