How are the images of both eyes combined to form one image?
The visual system contain several stages in which the information from both eyes is progressively mixed and analyzed, starting in the lateral geniculate nucleus. 

Even if you feel seeing just one merged object with both eyes, your brain is using both images to calculate the 3D position of that object.

One fun fact: The image that you feel centric, it is actually bias towards one of your eyes, the called "dominant eye". You can check what is yours by a simple experiment