Did anybody ever conduct experiments on fusing living brains?
The odds of that are really very slim. "Growing brains" in vitro is rather difficult, and it appears that no one has created the brain of a real organism, but they have created "brains" from cultured dissociated neurons.
Fusing brains is complicated by the fact that you need to make the appropriate synaptic connections between neurons, and what exactly are the appropriate connections is still being worked out. As a side note, this is one of the goals of the BRAIN Initiative.
The closest I believe people have come to anything approaching fusion are brain transplants in non-human animals. This might be the closest thing to an in vivo fusion, especially if it is only a partial brain transplant. This is discussed a bit here.