Heterogeneous solid electrochemical cells that contain two mixed ionic-electronic conductors (MIECs) exhibit quite unexpected I-V relations when the two MIECs conduct ionic defects having different absolute values of their effective charge z. For example, the larger voltage drop need not occur on the MIEC with the larger resistance to electronic (electron/hole) current. When the ionic transference number of one of the two MIECs is close to unity and its resistance to electronic current is large enough, then a simple relation is imposed on the ionic and electronic currents in the other MIEC. This is an ambipolar motion of ionic and electronic charge carriers with a non zero total current. The effect on the I-V relations of adding a solid electrolyte in series with a MIEC, conducting defects with different z, is discussed in detail, including the implications on ionic conductivity measurements.