A positive ion detector has been developed that is capable of simultaneously measuring two ion beams. It is made up basically of two scintillation detectors back to back. Ion beams enter a chamber on either side of a thin dividing partition. They are attracted to two electrodes at 40 kV where they release secondary electrons. A magnetic field directed along the axis of the incoming positive ion beam deflects the two electron beams so that they strike opposite walls of the ion detector. Two small organic phosphors situated in the walls at the electron impact points produce light signals that are recorded by photomultipliers. Focusing of the electron beams and the efficiency of detection for gas and heavy metal ions have been investigated. Measurements of tungsten isotopic ratios have shown that the stability of the system is such that the precision of measurement is defined almost entirely by the statistics on the number of ions counted.