How can we connect two brains to a video game by means of a BCI, and what will happen when we do so? How will the two users behave, and how will they perceive this novel common experience? In this paper we are concerned with the design and evaluation of multi-user BCI applications. We created a multi-user video-game called "BrainArena" in which two users can play a simple football game by means of two BCIs. They can score goals on the left or right side of the screen by simply imagining left or right hand movements. To add another interesting element, the gamers can play in a collaborative manner (their two mental activities are combined to score in the same goal), or in a competitive manner (the gamers must push the ball in opposite directions). Two experiments were conducted to evaluate the performance and subjective experience of users in the different conditions. In the first experiment we compared single-user situation with one multi-user situation: the collaborative task. Experiment 1 showed that multi-user conditions are significantly preferred in terms of fun and motivation compared to the single-user condition. The performance of some users was even significantly improved in the multi-user condition. A subset of well-performing subjects was involved in the second experiment, where we added the competitive task. Experiment 2 suggested that competitive and collaborative conditions may lead to similar performances and motivations. However the corresponding gaming experiences can be perceived differently among the participants. Taken together our results suggest that multi-user BCI applications can be operational, effective, and more engaging for participants.