Unix utility sed can be used serve the purpose
Assuming file1.txt and file2.txt are two input files and
file3.txt is the third file.

Contents of the file1.txt 
line 1
line 2
line 3

Contents of the file2.txt 
LINE 1
LINE 2
LINE 3


After executing below mentioned command 

sed -n -e '{
> R file2.txt
> p
> }' file1.txt > file3.txt


The third file file3.txt will be 
line 1
LINE 1
line 2
LINE 2
line 3
LINE 3