I wouldn’t call that “split content” so much as displaying posts in a columnar fashion — which is not hard to accomplish if you know what you’re doing in PHP.
Your first step is to set up a two-column layout for the posts section on your blog using <div> tags and CSS. If you’re not knowledgable in this area, you’ll find numerous tutorials online. One good place is A List Apart. The Codex has a number of other resources:
http://codex.wordpress.org/CSS
Next (actually, finally) is the PHP handling the ‘split’ for your columns. A simple way to accomplish it is to insert a counter that ticks off every post, then tests for when it hits a certain number (and when it hits this, closes your first div column and opens the second).
For an example we’ll assume you have 10 posts displayed and want 5 in each column. Initially we want to begin before The Loop your columns’ “holder” div along with the first column’s div:
<div id="columns">
<div id="column-left">
<?php while(have_posts()) : the_post(); ?>
Note the id’s here are examples. Use whatever you want; you can choose to use classes instead of unique id’s, as well.
The code to count off posts is slipped in right after the start of The Loop:
<?php while(have_posts()) : the_post(); ?>
<?php $post_counter++; ?>
Then we want to add the counter test and div insert after the counter but before any post content displays:
<?php $post_counter++; if($post_counter == 6) : ?>
</div>
<div id="column-right">
<?php endif; ?>
Once The Loop ends you also need to close up your divs:
<?php endwhile; ?>
</div>
</div>
Now the example in full:
<div id="columns">
<div id="column-left">
<?php while(have_posts()) : the_post(); ?>
<?php $post_counter++; if($post_counter == 6) : ?>
</div>
<div id="column-right">
<?php endif; ?>
~ Post content goes here ~
<?php endwhile; ?>
</div>
</div>
