Can two keyboards be used to play split-screen multiplayer?

Yes.

To enable the 2-keyboard layout, follow these steps:

1. Plug both keyboards into your Mac.

2. Double-click on the Sonic & SEGA All-Stars Racing application icon while holding the Command (⌘) key down. The Sonic & SEGA All-Stars Racing pre-game Options window will appear.

3. Select the Extras tab.

4. Check the Use 2-keyboard layout checkbox.

5. Click the Play button to start playing.

Enabling the 2-keyboard layout will alter the keyboard controls for Player 1. The controls for both players are as follows:

Player 1

• Left (◄) - Steer Left
• Right (►) - Steer Right
• Down (▼) - Fire Backwards
• D - Accelerate
• S - Drift / Trick
• Left Shift (⇧) - Look Behind
• A - Use Item / Sound Horn
• Enter (↵) - Pause

Player 2

• , - Steer Left
• . - Steer Right
• / - Fire Backwards
• E - Accelerate
• W - Drift / Trick
• Q - Look Behind
• Space Bar - Use Item / Sound Horn

• Backspace (←) - Pause

  
  NOTE: Both sets of keyboard controls work on both keyboards. Make sure to only press the keys for your set of keyboard controls, or you'll disrupt your opponent's game. You wouldn't want that...