Sonic Mania is the game you play; Little Nightmares runs beside it as a required companion.
Player 1 is Sonic in Sonic Mania. Player 2 is the real Six, played in Little Nightmares. Two players on one PC.
Press F9 in either window to switch between Sonic and Six, so Sonic and Six are one keypress apart instead of an alt-tab apart.
On start, a banner across the top of Sonic Mania names the two players and where to look; after it fades a dread panel stays in the top-left showing the dread level, the stage, the two heroes and the Nightmares level you are in. It is drawn inside Sonic Mania's own frame by a Direct3D 9 overlay, so no external mod loader is needed.
Slow-build dread: the screen tint deepens through CALM -> UNEASE -> HUNTED -> THE HUNGER as your Nightmares progress rises.
Dread is computed from your own Little Nightmares save (its progress markers and current level). Nothing from either game is copied or shipped; the package is one original d3d9.dll.
Local play only. There is no online, LAN or networked multiplayer. Melty starts both games on your PC and you share the keyboard with a second player.
Play from Melty: it installs the overlay into your Sonic Mania folder, points it at your Little Nightmares folder, and starts both games.