Is it possible to stack up two extenders 1.4x Mk2 + 2x Mk2 on a canon 70-200mm f2.8 L IS lens with Canon 5d Mark iii ?
With the recent firmware update i reckon its possible to autofocus with the center point @ f8.
not the genuine canons
As stated above, the 'genuine Canon' TC's the OP lists can be stacked - they are the MkII versions. The 2xIII has two additional elements compared to the 2xII, and there's not enough space at the back of the rear element to accept the protrusion of the 1.4x TC.
Note that you can stack a 1.4xIII and a 2xII - the limitation is for the 2xIII.