Research in gut flora and fauna shows that they have a huge impact on your lifestyle. In a very, very simple depiction:
Gut flora and founa comes in a wide variety. They grow better with some ingredients than with others. If you eat fast food burgers all the time, certain gut flora and fauna grows more abundant than others. To stay so successful, they use hormones to influence your brain in subtle ways, so you keep in a certain routine and keep getting the same foods. The flora and fauna even reward the person via hormones for the behaviour. This is why it is difficult to change a diet and some behaviours. It takes a while before a new group of flora and founa is most abundant in the gut to overpower the previous strong group and make a routine. Not to mention that the brain neurons grow into a certain structure to make such behaviours and routines easier.
This is not just gut flora and fauna. You can see many sicknesses and even parasites also influencing the brain. For example, they try to get you to avoid water (rabies), or get friendly with cats (cat parasites in a mouse and humans), or find a high place to be spotted by birds and being eaten or just to spread seeds (Cordyceps fungal infection on snails and some insects respectively).
What one brain thinks the other does
Now you migh have thought it is important to have a direct connection. One brain controls every action and emotion. This connection might be possible, but is much more complex than it needs to be. The parasitic brain can work from a higher 'plan'. You do not give the main brain every action, but a goal to strive towards. You make someone fear water if you want them to stay away from it, instead of actively taking the reigns to turn around and walk away.
This achieves two things. On a global level the parasitic and main brain are in sync. Their goals are aligned thanks to the subtle changes in the Hormones (and eventually the synaptic cleft of a neuron). In a way they think the same, even if the main brain is unawares of any changes and does not actively communicate back. The main brain is rewarded for certain processes, routines and behaviours, while subtly punished for others. Everything seems to be the main brains idea, like going to , despite that it is a carefully constructed hormonal soup that eventually changed even the brain makup to enhance those behaviours.
The second reason is speed. The main brain is unhindered by the parasitic brain for choices. It knows the plan and can act immediately. It does not need to consult and the parasitic brain, now wait for a reply. This isn't going correctly at all times, but in general it should work to the advantage of the parasitic brain.
You could argue that it isn't full control, but it seems to me that it's the most plausible. Primitive but super effective forms are already in nature. The only other option I see is just implanting neurons directly from brain to brain,but that seems far fetched, neigh impossible and dangerous to attempt for many neural reasons.