It was a feature that never made it into Banjo-Tooie. There were some secret areas in Banjo-Kazooie, that when Rare finally released Banjo-Tooie, you'd be able to access those areas. They'd probably have done it by having Banjo-Tooie prompt you when to switch Banjo-Kazooie into the cartridge slot. But even to this day, the locked areas in Banjo-Kazooie remain... locked!
Would love to see the interview, but my computer is too crap to run videos. According to the blurb, they give more details on Banjo-Threeie, and talk more about Viva Pinata.
Viva Pinata looks like great fun, so it'd be nice if they were planning a sequel.