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Sonic 3 and Sonic & Knuckles combine to create Sonic 3 & Knuckles when you insert one game into the other. This was all thanks to SEGA’s lock-on technology, except that technology was actually kind of a sham. You see, Sonic 3 & Knuckles was originally one game but was too big to fit on one cartridge. So SEGA split the game up into two parts, with the data for Sonic and Tails along with the first half of the game on one cartridge and the data for Knuckles along with the second half of the game on the other. Connecting the two would just give you access to the full original game. Connecting the game to Sonic 2 would simply cause Sonic & Knuckles to load Sonic 2’s stage data along with the Knuckles character data. This is why Eggman wouldn’t be wearing his robot mask in the Knuckles version of Sonic 2. This is also why no other game worked with Sega’s supposed lock-on technology.