1. Sort Files

   sort Name.txt -o Name.txt
   sort Firstname.txt -o Firstname.txt

2. Merge the two files on the first column

   join -o 0,2.1,1.2 Name.txt Firstname.txt > User.txt

3. Results should be like this

1 John Smith
2 Jane Doe
3 Jung Kim
3 Un Kim
4 Mickey Mouse