Sure, you can play this game alone as well. You can control the first character (Fireboy) with the help of arrow keys (← → ↑ ↓) and second character (Watergirl) with the help of A W D buttons, but I believe that the game is more entertaining when you play it with your friends. One play can control one character and another one second character, you can overcome obstacles together, complete levels together and even compete with each other (for example player which reaches the finish line first has better gaming skills).
You can call it a multiplayer option at some point, it is really interesting one.