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