What is about the electromotive force of the cell?

The electromotive force of the cell, may be sum or difference of two electrodes potentials. If both electrodes are above SHE or both are below SHE then electrodes potentials are subtracted to get the e.m.f. If one of electrodes is above SHE and other is below SHE, then the electrodes potential are added up to get e.m.f.