In Sonic 2 for Sega Genesis, go into the options menu, and using the 'Sound Test' option, enter the following code: 73, 6C, 65, 6E, 64, 65, 72, 6D, 61, 6E, 00.
After doing this, hold A, and press Start to soft-reset the game. Skip the various screens, keeping A pressed (not unlike unlocking the level-select screen) for the entire duration until you select '1 Player'. Please note that there will not be a confirmation noise, so if nothing abnormal occurs, power-down your console, and try again. If it still doesn't work, it's possible that your copy is not the right version. So far, only revisions 1.05 & 1.07 allow for the encounter.
v1.10 removed the "easter egg" which was, in fact either inaccessible, or absent altogether from the day one version of the game. It's unknown why this was added to the game; it's horrifying, and doesn't add any value to the game, aside from the novelty of hard-erasing Sonic from the cartridge. The summoning ritual doesn't seem to work when you try to play as Tails alone.