okay lets think about this. you have two boxes (books) on top of each other. The bottom face of the top book = 1 polygon, the top face of the bottom book = 1 polygon. 1+1 = 2.

Now you want to cut the top and bottom face of the bottom and top books (for what purpose I do not know.

But in the process you will create 4 triangles on the bottom and top face of the top and bottom books or eight new 3-sided polygons where you originally had only two 4-sided polygons and create bad geometry for the rest of the object unless you carry the cut edges all the way around the combined geometry. Fixing the geometry from teh whole new object and carrying the edges you cut all through the part will actually add a lot more then the 8 triangles on just the top and bottom faces in question.

Not to mention you also create a new problem for yourself when you go to texture these books because now they are combined and one is twisted and the bottom and top faces of the them are now combined in an odd way.

So what are you hoping to accomplish by doing this? You are not going to save polycon and you are only going to make texturing more difficult?

I mean it's not hard to combine them and I can show you how but in the end I see no real purpose in it other them clutter the geometry, add more polys, complicate texturing, and burn time better spent other other tasks.