Banjo-Kazooie had the Stop N Swop feature, but it was never utilised in Banjo-Tooie, being one of the people who painstakingly collected all of the secret items to discover that they did nothing I've been wanting an explanation for 8 long years. 

Stop N Swop was supposed to sllow players to take out their Banjo Kazooie cartridge and whilst leaving the power on, put in Banjo Tooie for additional content.

Rare announced that certain areas in the original could only be accessed by meeting requirements in the sequel, and even had a menu containing 7 items that did absolutely nothing: 6 differently coloured eggs and an ice key.

Although code for Stop N Swop was included in Kazooie, it unfortunately was scrapped in Tooie. Both Nintendo nor Rare had since discussed reasons why it never came about, yet it's been referenced in a fair few Rare games developed since then: Grabbed by the Ghoulies, Banjo Pilot etc.

MTV Multiplayer spoke to Salvatore Fileccia, lead software engineer at Rare, on the upcoming third installment in the Banjo series for Xbox 360, and popped the question.

In 2004 Rare filed a patent revealing Stop 'N' Swop would be accessed by using the N64's Rambus memory solution to temporarily retain information.

Fileccia reiterated the reason Nintendo dismissed the idea: N64 hardware revisions. Older systems would have allowed for up to 10 seconds for players to swap over the games, whilst newer models granted only one second to perform - leaving newer N64 adopters with more risk to damaging their consoles.

That said, it's likely there still may be more reasons as to why Stop N Swop was cancelled, and Banjo-Kazooie: Nuts & Bolts could be the perfect outing to disclose everything.

Source.