Tandem wing aircraft with a tail are still largely not explorded enough ..people have this idea of a cannard and efficiency so if a tail is added all intrest seems to be lost ..the idea of a tandem wing is to spread the load over a wider area and still use a tail as normal single wing aircraft do ..a biplane uses the tail the same way as a single wing aircraft whether it is a stagger wing or not so a tandem wing is just a stagger wing biplane and the tail still opperates the same so no cannard concept comes into it .the c/g is between the two wings and the tail is still a half wing span back with normal surface area calculations so when the c/g reaches its aft limit that is the same as single wing aircraft .the tail is sized so it has no lifting camber and is a symetrical airfoil like normal single wing aircraft and the aft c/g limit is placed at where the aircraft will tail slide and not recover ..where as people think tandem wings are cannards and so are vunarable to tail slide .just think of a triple wing air craft not as a cannard but the two forward wings act together and the tail acts as an elevator and c/g is the mid way point between the two forward wings .