// initiate the default wordpress loop as usual
<?php if (have_posts()) : while(have_posts()) : 

// set a count variable to increase with each loop
$i++; 

// test the variable modulus against a zero value
// odd values return true, even values return false
// see proceding discussion for more information
if(($i % 2) == 0) : 

// skip to next post if variable is an even number
$wp_query->next_post(); 

// display the post if variable is an odd number
else : the_post(); ?>

// open a division for the left column
<div id="left-column">

// display the title of the post
<h1><?php the_permalink(); ?></h1>

// display the post content
<?php the_content(); ?>

// close the division
</div>

// close the first if statement
<?php endif; 

// close the loop
endwhile; 

// if there are no posts that meet the criteria
else: ?>

// display some alternate content
<div>Alternate content</div>

// close the second if statement
<?php endif; ?>

// reset the count variable to zero
<?php $i = 0; 

// reset the loop
rewind_posts(); ?>

// the second loop is essentially the same as the first
// the only difference is that we are testing the count variable against a non-zero value to display only even numbers
<?php if (have_posts()) : while(have_posts()) : $i++; if(($i % 2) !== 0) : $wp_query->next_post(); else : the_post(); ?>

// and of course a unique div id for the right column
<div id="right-column">
<h1><?php the_permalink(); ?></h1>
<?php the_content(); ?>
</div>

<?php endif; endwhile; else: ?>
<div>Alternate content</div>
<?php endif; ?>