There is also another command 'paste'.

Example: Created two files (1.txt and 2.txt)

[efg@fedori ~]$ cat 1.txt

file1: line1

file1: line2

[efg@fedori ~]$ cat 2.txt

file2: line1

file2: line2

In order to merge the lines of these two files we can use paste:

[efg@fedori ~]$ paste 1.txt 2.txt

file1: line1 file2: line1

file1: line2 file2: line2

Also check out 'man fold'