You should also be able to do something like this in your content loop using HubL for your blog to add in an item after the 2nd listing item:
{% if loop.index == 2%}
<div class="post-item grid-item">ADD AFTER 2nd ITem</div>
{%endif%}
You would then need to use CSS to make sure your grid is positioned correctly. Note, that if you are using paginated content, the above would add this on every page where the loop outputs. If you only wanted it on page one, you would also have to include a conditional to take the current_page_num variable into account.
Hopefully, this helps point you in the correct direction.