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