Why do we study under one teacher?
We can see Milarepas story. When Marpa would not give a discourse Milarepa went to another teacher. However with this teacher he could not gain any experiences.
Is this saying that this teacher Lama Ngokpa was not teaching correctly? NO. He was a perfect teacher. The issue is that Milarepa had given his heart to another teacher. Because he had bonded deeply with this teacher the bond could not develop with another teacher.
The first time you fall in love is always an experience you cannot get a second or third time. There is always one person in your life you develop a deep bond with one that you cannot feel for others. But then there is the question of Atisha and Je Tsongkhapa who had many many teachers. Atisha had a deep bond with Tara. He didnt ask many people for advice he always asked Tara.
For some people it works to have one teacher for some people many. So what is the answer?