Split-screen multiplayer modes (or just some multiplayer modes in general depending on the game) actually are more demanding than regular single-player modes. This is nothing out of the ordinary (Dolphin basically has to process more things, e.g. input, sound, graphics, game logic).
You might need to overclock your CPU to account for multiplayer on MKWii. Try raising your clock in increments and see which is stable and runs MKWii at decent speeds for multiplayer. Of course, see if you can run this game's 4-player mode on stock before attempting anything.
You might need to overclock your CPU to account for multiplayer on MKWii. Try raising your clock in increments and see which is stable and runs MKWii at decent speeds for multiplayer. Of course, see if you can run this game's 4-player mode on stock before attempting anything.