I've seen this asked numerous times in the discussions so I'm just putting this here to save people asking:

From the main screen open the settings menu. Default keyboard key is i.
Select Game Settings.
Set Number of Players on Keyboard to 2.
Select Close and when it asks to restart the game choose Yes.
From the main screen open the settings menu again.
Select Button Settings.
Press right Shift to open the player 2 keyboard buttons.
Now on the player 2 side change all the arrow keys to something else.
This will free up the arrow keys so you can bind them to player 1.

This also works for the Numpad.