Software developer Takayuki Shimamura said Namco Bandai was asked to a lend a hand because everyone at Nintendo had "strong feelings" about the project and felt compelled "to go all-out."
From the sound of it, Namco Bandai provided the bulk of the development muscle behind the title, while Nintendo handles logistics, managing everything from planning to localization and network code. The collaborative effort proved especially fruitful with regard to online multiplayer in Tennis.
"At first, we were uneasy about whether we could really do it, but we tossed around all kinds of ideas with Namco Bandai Studio," confessed coordinator Kozo Makino.
Sending complex information from gyro sensors and accelerometers in Wii Remote Plus controllers with up to four players on the court at once posed quite the challenge.
"We then decided to test some of the ideas, and the one we eventually adopted was an idea suggested by a communications programmer at Namco Bandai Studio," Shimamura said. "Thanks to that, we were able to pull it off."
Iwata Asks: Wii Sports Club [Nintendo]