Sometimes there are good reasons for a second therapist, such as to address a specific issue (eating disorder, or something) that the first one doesn't have experience with, while the first is helping you with everything else. It is very important that they do know about each other and can communicate to coordinate your treatment though. Otherwise there can be a problem with triangulation, where you can get into a pattern of playing one off of the other, and that doesn't help anybody.

It's true that some people have two parents. Usually the two parents know about each other and communicate with each other about how to raise each child, and what is going on, and what the child needs at a particular time. Sometimes the parents don't communicate or have different ideas on child-rearing. Triangulation happens, and usually that is a big problem. The idea is not to replicate dysfuctional family issues, or create them, with two therapists.