田中孝司 is pronounced as たなかたかし and たなかこうじ. I'm afraid that there could be other pronunciations of the same kanji characters.
How should I understand it?
If a name can be read in several different ways there is no way to predict how it is read and the only reliable method is to ask the owner of the name.
In the case you brought, it seems that the two reading you gave are the only ones in use according to this dictionary of names.
However, the name you mention happens to be the name of a football player and in that case his name is 田中｛たなか｝孝司｛こうじ｝.