The most common form is "responsive dialogue." Usually, one person is the main speaker, who is responsible for throwing out the punch lines. The other one lends support.
最常見的形式是「對口相聲」，通常由一個人主講，稱之為「逗哏的」（「哏」就是笑話）；另一個演員幫腔，稱之為「捧哏的」。