The two column posts look great when the text is the same, create a post with different sizes of words and you might find that the posts get out of line.
If you look at the page the two columns widths do not align with the header image, this is due to the right margin in the second column.
[Post 1]-[Post 2]-
[Post 3]-[Post 4]-
I use a different class for the left and right columns, and this makes the posts line up nice across the page and where the text is shorter or longer, clear both on the left and no margin on the right.
Then in the content or loop I add a counter variable.
<?php $counter = 0; ?>
<?php while ( have_posts() ) : the_post(); ?>
<?php $counter++; //Increment the Counter ?>
<?php if($counter % 2) : ?>
<?php else: ?>
<?php endif; ?>
<!-- Post Output Code Here -->
<?php endwhile; ?>
Just thought I would add this comment as you might want to revisit the layout with different content values.