Even / Odd row skipping

System

I am generating a two column table where we would like to populate left column first and then the right column.

One way I thought of how to do this is to put the odd numbered rows in the left column and then the even numbered rows in the right column. All of this is within an interate statement. Since there are 20 possible replicants, how can I build a loop to check for odd/even rows without having to code in a bunch of case checks. On top of that, there will be 12 sections with the similar replicants.

The code snippet below works to skip row two, now I'd like to do all even rows from 2 to 20.

<iw_perl>$n++;</iw_perl>
<iw_if expn='{iw_value name="$n" /} eq "2" '>
<iw_then>
<iw_next></iw_next>
</iw_then>
</iw_if>
... if not skipped, place the result into the table