With one mic, take turns freestyling and then passing the mic to the next person. Part of the objective is to learn about coordinating how long the first person will be on the mic and when the other person is ready to take over. This means holding onto the mic long enough that the other person has a chance to get ready for it, but not so long that they get impatient. You will develop style and skill at passing a single mic between two rappers so the pass doesn’t look or sound fumbled, and the lyrical gap between freestylers will be minimal or non-existent. Both people need to be ready and cooperate. Ideally, the end of one freestyle and the beginning of the other will be complementary as well.
You can practice with two or more people and one mic, but it’s also useful to practice with two or more mics. With two mics, the goals are the same, but it’s a little easier. You don’t have to physically pass the mic before the other person can begin, but you still have you pay attention and cooperate.