Bias can appear both in what facts they decide to cover and in how they cover it. The first part is the one people usually ignore and the easiest to hide. You can just fact check one person on their every sentence and not fact check the opponent on their every sentence. You can also decide to only fact check mostly true or true statements for one person and mostly false or false statements for the other person. You'd be performing proper fact checking most of the time, but you'd be helping one person more than the other.