Also for fun, a Perl 6 version.
Note that this only works for files of the same length, because Z terminates (without error) when the shorter list ends. 

use v6;
sub MAIN ( $in_path1, $in_path2, $out_path = 'CONCATENATED_FILES' ) {
    my $in1 = open $in_path1, :r;
    my $in2 = open $in_path2, :r;
    my $out = open $out_path, :w;

    for $in1.lines Z $in2.lines -> $line1, $line2 {
        $out.say: "$line1\t$line2";
    }

    .close for $out, $in1, $in2;
}

[download]

In reply to Re^2: Use 2 files and print each line of each one side-by-side by Util
in thread Use 2 files and print each line of each one side-by-side by Anonymous Monk

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