A system is disclosed which allows two image signal sources with different update rates to timeshare a single calligraphic display monitor, such that the resulting image is a combination of the image data from both sources. The image signals from one source are monitored and analyzed by the system controller, which is adapted to locate temporal locations of the correct duration and frequency in the update cycle when the first source is not writing to the monitor. The controller controls the triggering of write signals by the second source to occur in the located temporal location in the write cycle of the first source. The controller also controls an analog switch for switching the image signals to the monitor during the times when the second image source is writing to the monitor. By controlling the timing of write signals from the second source, the two sources are maintained in the correct phase relationship with no overlap of write signals.