a storage capacity required to store the first read data during the time period from {TAIL ~ the time} when the leading end of the document sheet reaches the reading position of the first image reading unit to the time when {HEAD ~ the trailing end} of the document sheet passes through the reading position of the second image reading unit 
a storage capacity required to store the first read data during the time period from { TAIL ~ the time } when the leading end of the document sheet reaches the reading position of the first image reading unit to the time when { HEAD ~ the trailing end } of the document sheet passes through the reading position of the second image reading unit
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH