a storage capacity required to store {HEAD ~ one page} of {TAIL ~ the second read data output} from the second image reading unit; and 
of
a storage capacity required to store { HEAD ~ one page } of { TAIL ~ the second read data output } from the second image reading unit ; and
OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH