Легко можно сделать с awk
awk '
/^!!/{ #for line stared with `!!`
getline <"file2.txt" #read 1 line from outer file into $0
}
1 #alias for `print $0`
' file1.txt
Другая версия
awk '
NR == FNR{ #for lines in first file
S[NR] = $0 #put line in array `S` with row number as index
next #starts script from the beginning
}
/^!!/{ #for line stared with `!!`
$0=S[++count] #replace line by corresponded array element
}
1 #alias for `print $0`
' file2.txt file1.txt