[2.2.13] Selecting a "ABC & XYZ"Character via Level Select and loadng another level with that will change Player 1 into Sonic.
Sonic & Tails is the vanilla duo to play as without addons, though some addons add in the option to play as ABC & XYZ. If you select these addon characters via Level Select and then load another map that way... the first player will change into Sonic.
Addons that I have tried as these were the ones I can remember currently...
Blaze & Marine (RushChars)
Mario Bros.
Eggpack & Eggette
(Need both addons to have the option to select both)
Dirk & Whirl
(Team Kinetic)
Video: