You can use the paste command as:
paste file1 file2
Demo:
$ cat file1 1 2 3 $ cat file2 3 4 5 $ paste file1 file2 1 3 2 4 3 5 $
The default character used as the delimiter is the tab. If you want some other character to say a space, you can use the -d option as:
paste -d ' ' file1 file2
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