First post full width, rest in two columns

Here is my loop

<?php if ( have_posts() ) : ?>

        <?php /* Start the Loop */ ?>

<?php $i = 0; while ( have_posts() ) : the_post();  ?>
    <?php if ($i++ == 0) : ?>

        <div class="entry-content-main">
        <?php get_template_part( 'content', get_post_format() ); ?>
        </div>

    <?php else: ?>

        <div class="entry-content-rest">
        <?php get_template_part( 'content', get_post_format() ); ?>
        </div>

    <?php endif; ?>
<?php endwhile; ?>

and my css

.entry-content-rest {
    width: 48.5%;
    float: left;
    border: 1px solid #ddd;
    border-radius: 7px;
    }

This displays my content as follow

Post1

Post2      Post3

Post4      Post5

My problem is, say post 2 is longer (have more contents) than post 3, then my columns is displayed way wacky like this

Post1

Post2      Post3

open       Post4

Post5      Post6

Any suggestions to fix this

you could rewrite the full code and use the build-in loop counter $wp_query->current_post to fix the css classses and add a class for the first post per row to clear the float to prevent the ‘wacky sticking’;

example:

<?php if ( have_posts() ) : ?>

<?php while ( have_posts() ) : the_post();  ?>
        <?php /* Start the Loop */ ?>

        <div class="entry-content-<?php echo (( $wp_query->current_post == 0 ) ? 'main' : 'rest' ); if( $wp_query->current_post%2 == 1 ) echo ' left-post'; ?>">
        <?php get_template_part( 'content', get_post_format() ); ?>
        </div>

<?php endwhile; ?>

additional css:

.left-post { clear: left; }