Bash (203203, 197 chars)
#!/bin/bash
r=" ▝▘▀▗▐▚▜▖▞▌▛▄▟▙█";while▝▘▀▗▐▚▜▖▞▌▛▄▟▙█" IFS=IFS=;while read -r m;do IFS= read -r n;forn o;for((l=${#m}>${#n}?${#m}:${#n},i=0; i<l; i+=2)){
printf -ve %-2s "${n:i:2}" "${m:i:2}";e=${e//[^ ]/1};o+=${r:2#${e// /0}:1};};echo "$o";o=;done"$o";done
or
r=" ▝▘▀▗▐▚▜▖▞▌▛▄▟▙█"
while IFS=
while read -r m; do
IFS= read -r n o
for ((l=${#m}>${#n}?${#m}:${#n},i=0; i<l; i+=2)) {
printf -ve %-2s "${n:i:2}" "${m:i:2}"
e=${e//[^ ]/1}
o+=${r:2#${e// /0}:1}
}
echo "$o"
o=
done