1. A color image processing system comprising: a) a first reading apparatus for converting a first object color image into a first color image signal which is a plane-sequential color signal; b) a second reading apparatus for converting a second object color image into a second color image signal which is a plane-sequential color signal; and c) synthesizing means for synthesizing said first and second color image signals, wherein said synthesizing means synthesizes respective color components of the first color image signal output from said first reading apparatus with respective corresponding color components of the second color image signal output from said second reading apparatus; and d) control means for controlling said first reading apparatus and said second reading apparatus based on a synchronizing signal for each color of said plane-sequential color signal of said first reading apparatus or of said second reading apparatus.