aaaaaa
bbbbbb
!! 1234
!! 4567
ccccc
ddddd
!! 1111

first line
second line
third line

aaaaaa
bbbbbb
first line
second line
ccccc
ddddd
third line

awk '
    /^!!/{                    #for line stared with `!!`
        getline <"file2.txt"  #read 1 line from outer file into $0
    }
    1                         #alias for `print $0`
    ' file1.txt

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

$ perl -pe 'BEGIN{ chomp(@a=`cat file2.txt`) } s/^!!.*/$a[$i++]/e' file1.txt
aaaaaa
bbbbbb
first line
second line
ccccc
ddddd
third line