Sonic & Knuckles / Sonic the Hedgehog 2

Sonic & Knuckles features “lock-on technology” that allows players to open the hatch on the cartridge and insert a second cartridge. When Sonic 2 is inserted, Knuckles the Echidna in Sonic the Hedgehog 2 is unlocked, wherein the player can play through Sonic 2 as Knuckles with all of his abilities.