Co op in fable 2... does it need to be 2 different xbox's? or can you do it on the same xbox?

my girlfriend and i enjoy playing fable 2, we know there is a co op mode but do not know how to access it, does it need to be 2 seperate xbox's or since we live together can we just use the one xbox we own to play at the same time with each other? if so how?