You can't throw a soda at your second player unfortunately, so you'll have to throw it at an enemy during a level.
The easiest way is to go to The Lost World level "InGen Arrival" where at the beginning there are enemies to fight. Pick Dennis Nedry as your character (the cheat to unlock the Hawaiian shirt version of his character for free is RAVKRT). Just hit any character with a can by pressing and holding
to aim at an enemy and he'll throw it. You might want to walk away from the crowd in order to aim better.
Here's a video (not mine):