Solution: merge two files / append two files in Linux

Code:

cat file1 file2 .. fileN  > mergedfile

Note the '>' in the command: it points that the target file follows, don't forget to put it there.

Advance:
 
If you accept that entries in the resulting file are sorted, you can use uniq to remove duplicates.

Code:

cat file1 file2 | sort | uniq > mergedfile