Hi Peter, 

thanks you for your interest, sorry for not explaining it clearly enough.

So okay the black box was just my way of showing you guys the region where the heat exchange takes place. so actually there is no body between the two rectangles. So yes, it can be assumed that except for the black region there is no heat flux between the bodies. 

The region represents a small x-distance in the bottom body where the connection between both rectangles takes place. 

And what i want to simulate is, if theres a heat generation at that region, how the temperature rises and is distributed inside the bodies.